In my optiplex 7## SFF I have a Dell 2.5” adapter caddy that looks like this one.

Yes, you can use 6Gbps SATA III drive, as it's backwards compatible with SATA II.  It will still speed up that 755 quite nicely.

When I had a 755 MT, I got many wonderful suggestions when it comes to loading SSD.  So many in fact, I couldn't use them all if I wanted to.

I going to suggest do a fresh load onto the SSD, whether from USB or DVD.  I have DVD.  We just got done helping somebody with a 380 whereas cloned SSD wouldn't boot.  It could've had something to do with Legacy BIOS, which the 755 has.  Generations newer than the 755 and the 380 don't seem to have as many problems with cloned drives.

Dell doesn't support 3rd party cloning software, but that doesn't mean it doesn't work.

You can replace the HDD with an SSD and clone the HDD to the SSD using a program such as Macrium Reflect Free. You can use a SATA 3 drive but the 755 is SATA 2 I believe and any SSD is backward compatible. 

This is a nice adapter on Amazon.

Careful of cheapy adapters.  If they don't position SSD right, tab that hangs down from original caddy gets in the way of connections.  Adapters still go in the original caddy.

Mount the SSD in a silverstone SDP09

https://www.amazon.com/SilverStone-SDP09-2-5-Inch-3-5-Inch-Hot-Swappable/dp/B0049MPQDG

• Fits in any 3.5"drive bay with SATA interface
• Compatible with any 2.5"device
• All metal construction
• 1.0 mm SPCC with nickel plating

Set sata operation to AHCI and use OEM system builder DVD.

https://www.newegg.com/microsoft-windows-10-home-64-bit-installation-recovery-disc-only-no-license-key-included/p/N82E16832350237

Generic Key can be used to install but not to activate

Generic Keys
Windows 10 Home

YTMG3-N6DKC-DKB77-7M9GH-8HVX7

Windows 10 Pro

VK7JG-NPHTM-C97JM-9MPGT-3V66T

the 755 has AMT HECI drivers and the audio drivers are whql.

https://dl.dell.com/audio/R164900.exe

Intel AMT HECI & AMT SOL/LMS Hardware. 

The Current Driver for this works with Windows 7, Vista, XP, 32 and 64 bit.

One Size Fits all.

Operating Systems

Windows 7 32-bit

Windows 7 64-bit

Windows Vista 32-bit

Windows Vista 64-bit

Windows XP

Windows XP x64

Chipset

ftp.us.dell.com/.../R174616.exe

AMT-HECI

ftp.us.dell.com/.../Intel_AMT-HECI_A02_R255437.exe

AMT-SOL

ftp.us.dell.com/.../Intel_AMT-SOL--LMS_A02_R255438.exe

Hard Drive Installation (via WinZip) with Setup.exe File for Intel_AMT-HECI_A02_R255437.exe

Download

1.Click Download Now, to download the file.

2.When the File Download window appears, click Save (Windows XP

users will click Save) this program to disk and click OK.  The Save In:

window appears.

3.From the Save In: field, click the down arrow then click to

select Desktop and click Save.  The file will download to your desktop.

4.If the Download Complete window appears, click Close.  The file

icon appears on your desktop.

Install

1.Double-click the new icon on the desktop labeled Intel_AMT-HECI_A02_R255437.exe.

2.The Self-Extracting window appears and prompts you to extract or

unzip to C:\DELL\DRIVERS\R255437. Write down this path so the executable (I.e.

Setup.exe) file can be found later.

3.The Self-Extractor window appears.

4.Click OK.

5.After completing the file extraction, if the Self-Extractor

window is still open, close it.

6.Click the Start button and then click Run.

7.Type C:\DELL\DRIVERS\R255437 in the Open textbox and then click OK.

8.Follow the on-screen installation instructions.

Thanks speedstep, very thorough post. It has Win10 installed, I don't know if it's Home or Pro. I can find out later today.

Any special considerations for that OS such as BIOS config changes, upgrades or OS drivers needed? Most of the drivers listed above are for older OSes.

Ideally I'd like to clone the drive so I don't have to mess with OS install and activation. The file system is pretty clean, must have been a fresh install before he got it. I'll try the Macrium product or maybe Acronis. 

I appreciate the help, thanks to all.

Speedstep mentioned to make sure in BIOS to "Set sata operation to AHCI" if not already.  Speedstep had plenty to say, so you might've missed it.

Other than that, whether you go from Win7 HDD to Win10 SSD, or Win10 HDD to Win10 SSD, nothing needs changing in BIOS.  The first scenario in this sentence I did in my previous 755.  As long as your Win10 was already working, your BIOS might already be on AHCI.

All good adapters were suggested/recommended.  I used and use the Orico one myself. 

Screengrab below of AHCI setting in Legacy BIOS:

Just use the Generic Home or Pro key to install.

Generic Key can be used to install but not to activate

Generic Keys
Windows 10 Home

YTMG3-N6DKC-DKB77-7M9GH-8HVX7

Windows 10 Pro

VK7JG-NPHTM-C97JM-9MPGT-3V66T

Windows 10 is not supported on this model and many others.  However it works fine except for the INTEL chipset, video drivers, oem Audio Drivers, INTEL Management Drivers, INTEL HECI AMT SOL drivers, and a few others like Flex bay card readers.  Everything else is on the windows disk.  Not supported does not mean not working.  10 works fine on these models. If you want support buy windows retail. That's the only way you get support from Microsoft.

It will ask for a key because the system is so old.

Since it already has 10 the license is not an issue.

The key is stored on Microsoft's servers not on your machine.

It will re activate automatically once you get online.

The only catch is you have to install Home or Pro that matches what is already there.

There is no difference between home and pro media whether USB flash or DVD.

All windows 10 media now installs almost all versions except for special cases like the long term support builds and enterprise and education.

AHCI is required or you will have to install the INTEL F6 mass storage driver during install which is a pain and not necessary.

https://www.dell.com/support/kbdoc/en-us/000148150/