I did some research and found out that the chipset in this system, the H61, simply doesn't support AHCI or RAID. There's no way to enable either for a reason. :(

Wish I had known this. The value of this system just really went down the tube if you intend to throw an SSD in there.

OK, I accomplished what I needed to do.  Here's what I did, and what I've found:

I managed to downgrade my bios version to MS-A04 while patching it at the same time for AHCI.  I also found that this did NOT cure my flaky boot issues.  Upon further investigation, I found that any of my USB devices that were “storage-capable” (Zune-HD, USB external WD drive) were causing the pc to stall at boot-up, even though USB devices were listed far down the boot order.  I ended up disabling any/all USB boot devices in the bios boot order since I’m not booting from USB and can easily toggle it on in the bios if I actually needed to.  I did NOT disable “Legacy USB”.  That would have been disastrous.  Ok, maybe not disastrous, but since there’s no PS/2 ports on this computer, I wouldn’t have any keyboard available during bootup.  I would have had to clear the CMOS just to get into the bios!  Anyway, that issue’s solved.  With no USB devices in the boot order, the pc boots fine to Windows from a cold start.

Now, as for the flashing process, here's what I did:

1. Got a copy of AMIBCP from this link via bios-mods.com:  http://www.sendspace.com/file/owfsdf (it's an .rar compressed file, so get a copy of 7-Zip or something similiar to uncompress it.  What I did was uncompress the RAR and then the enclosed secondary ZIP file contained therein, and saved the extracted AMIBCP files to a folder.  I then made a desktop shortcut to AMIBCP.exe.  More on that later.

2. You’re going to need to FIRST change a Windows registry setting to enable AHCI, or else you could end up with an “Inaccessible Boot Device” error if you try to boot with AHCI enabled in the BIOS.

1. HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
2. In the pane on the right side, right-click Start in the Name column, and then click Modify.
3. In the Value data box, type 0, and then click OK.

3.  Downloaded the MS-A04 bios for the Inspiron 620.  Since this was a lower version than the MS-A08 I already had installed, I had to use a special file "switch" to get it to run.  If I were to just click on the MS-A04.exe file by itself, I would have gotten an error message that the bios version was the same or lower than my current version, and hence, it wouldn't run.  So, the "fix" is to click Start/Run in Windows, clear the Run box if necessary of any text that's already there, and then drag the file MS-04.exe (or whatever previous version you need to run) into the Run window.  Position your cursor at the end of the MS-A04.exe file name with your mouse, add a space with the keyboard, and then type in (without the quotes) "/forceit”.  So, the file name in the Run box should say something like:

C:\Users\username\folder\MS-A04.exe /forceit      

Of course, "username" and "folder" are whatever your name and folder path are.  I'm assuming C: as your Windows drive.

Click OK.  The bios update screen will pop up waiting for you to commit to updating it.  Don't click the "Update" option at this point.  Just leave it sitting on-screen, as this is the time you'll be patching your bios for AHCI.  To do that, go back to the Run folder, clear the box of the MS-A04.exe text if it’s there, and just type in (without quotes) "&TEMP&" and click OK.  Your Temp folder will open.  Scroll down the folder list until you see the MS-A04 folder. Double-click on that folder to open it up.  What I did was drag the .bin file (MS-A04.bin) -- and dropped it on top of my AMIBCP.EXE shortcut on my desktop.  This opens the bios editor. You can, of course, use the File/Open option on the AMIBCP.exe program, but it will take you an extra step, since .bin files are not looked for by default.  You’d have to select “All Files” from the File/Open box to then “see” your MS-A04.bin file as an option to open.  So, why not just drag the file onto the shortcut?  No muss, no fuss.

Now, I just looked for the settings that user Litwell so kindly took screenshots of on page 2 of this thread.  You'll be making 5 changes he's highlighted in yellow.  Save the file back to the MS-A04 folder.  Now, all the while the actual bios flash window is still on-screen waiting for you to commit to flash it.  Now, click the “Update” button, and it will flash your patched bios. 

4.  Of course, a reboot is needed after flashing, and that’s the time you’ll want to go into the PC's bios/setup and set your drives for AHCI vs. IDE, now that you have that option.  You’ll see you have a SATA options menu item.  Save your changes in the bios.  Boot into Windows.  Shortly after booting, you’ll be prompted to reboot for changes to take effect. I actually took an extra step and found Intel’s AHCI Controller, since this is an Intel chipset on the Dell Inspiron 620.  I got that here:  http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=20110&keyword=%22AHCI+SATA+Driver%22&lang=eng  I installed it, and rebooted.  My device manager now shows my drives as SCSI drives – which they essentially are with AHCI.  These drivers come with an “Intel Rapid Storage Technology” Control Panel applet, which tells you the status of the drives, speeds, etc.  Mine is reporting 3 gigs/sec transfer rate, write caching enabled, etc.  There’s also an option to toggle on an LED light for the port to show you which one the drive is connected to.  I haven’t done anything with that. 

Bottom line:  Since my boot problems were not the result of the MS-A08 bios update, I can always patch that and reflash to that one.  I read the changes for that update, though, and it says this: “Fix system will have some fan noise with some particular situations.”.  Well, this is about the quietest pc I’ve ever had, so unless this is referring to the DVD drive spin up noise, I’ll pass for now.

Here's what my Device Manager looks like after:

• Updating registry to enable MSAHCI (see info above)
• Flashing Bios down to MS-A04 from A08 while patching MS-A04 for AHCI (see info above)
• Enabling AHCI in bios post flashing (see info above)
• Booting into Windows after flashing/rebooting when prompted to enable AHCI in Windows (see info above)
• Installing Intel Rapid Storage AHCI Controller/Drivers -- Note "SCSI" now  (see link above)

Great Thanxs for all of you!

Especially for ErwanLeBihan, PippoX0 and Slikkster!!!

I got AHCI running on a "Inspiron 620 (Mid 2011)" and Win7 64Bit, and it works perfect up to now!

I’m patched myself the latest original Dell Bios, which comes as a download named "MS-A10.exe"

I want to give a short wrap up of this huge thread, with the information which was useful for me and took me a while to get it. Also the verification for Bios version A10. All information is already spreaded in this thread. So if you don’t understand any step, you have to read the whole thread.

The patch is no real functional change. It just makes "hidden menus" accessible in the standard bios menu of your PC. The menu is obviously deactivated by Dell, because they presumably do not want to guarantee that it works a 100%. It was probably not tested when they put this PC model on the market in 2011.  

• The used bios editor is called "AMIBCP4.53" and I found it in the posting "Posted by PippoX0 on 28 Aug 2012 6:22 AM" http://www.sendspace.com/file/je91vr . As a second source for the feature (if the link fails) I created also a download link which contains just the tool: http://www.mediafire.com/download/v10mg66q4bzxgfc/AMIBCP4.53.zip
• In the posting "Posted by Slikkster on 14 Feb 2013 9:47 PM" I found the option "/forceit" for the original Dell Bios flasher (Background: The bios updater refuses to update, if the bios is already up to date)

1. Open the registry editor with command "regedit" on path "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci" and change the value       "Start" from "3" to "0".
2. Find your temporary folder: Open a dosbox and execute "echo %temp%" to print the path.(e.g. "C:\Users\stefan\AppData\Local\Temp" on my pc). Open it in the file explorer.
3. Get the latest bios from the dell support page (is an executable)
4. Just start it - nothing more! A folder called "MS-A10" will appear in the temporary folder.
5. Go to the temporary directory and copy the bios file called "MS-A10.BIN" ( a file with size 4096KB) to another place. Close the tool.
6. Open it with the bios editor (easiest way per drag&drop on the opened tool)
7. Make the 5 changes, described by Litwell "Posted by LitwDell on 1 Sep 2012 9:21 AM" in the screenshots. Save! Reopen and check your Change!
8. Start the original Dell flasher with the force option. e.g.: "C:\tmp\MS-A10.exe /forceit" (Slikkster gives advice for beginners in the post quoted above). If your BIOS is older than the one you are flashing, you can just start the "MS-A10.exe" with double click.
9. replace the original BIOS file in the temporary folder with your patched version
10. Press "update" in the Flasher tool and wait till finished.
11. Reboot, enter directly the BIOS (press F2) and change to "AHCI" mode!!! Start windows, reboot once again, measure your speed improvement, enjoy your new fast PC!

To compare your changes you can download my patched Bios:

http://www.mediafire.com/download/v85r7docamfx3c9/MS-A10_AHCI.BIN

If you use this file the risk is up to you!!!

By, and another Thanks to the guys who figured out this!

Stefan