This forum post may help though there is a bit of conflicting information in there. And be careful as building PCs can become addictive. Every time I build a new one I swear it will be my last...until I get the urge to build another.

You can find all the compatible cpu on user benchmark validated by users.

https://www.userbenchmark.com/System/Dell-OptiPlex-755/3939

Re: I wanted to add just a small SSD as a boot drive and keep the HDD for storage, but it is a SFF and that HDD in that big tray with a fan takes up a ton of room.

I believe there are only two sata power connectors of small psu for hdd and odd.  I think the power connector may be slim sata not directly compatible with sata ssd.  You may find a thin laptop style odd to 2.5 caddy and mount the new ssd there replacing the stock thin odd.  

Also, top CPU for the 755 is the Q9650 - used myself.  (Not listed in Tech Guide.)  There might be a mix of info on if it take a higher CPU or even a Xeon.  I wouldn't bother, part in due to age of PC and $ reasons.  My previous 755 with Q9650, 8GB RAM, and SSD was a dynamo.  It actually booted a few seconds faster than my current 7010.

Trying the wrong CPU in a PC can also inadvertantly bend pins.  Q9650 is a good deal on eBay.

Optiplex 755 Tech Guide 

The HDD bay, with an adapter, can fit both an SSD and a 2.5" HDD.  I think you'd get the best of both worlds.  You might not lose DVD drive.  Dell has a SATA data splitter cable:  https://www.dell.com/en-us/shop/dell-sata-cable/apd/470-acvc/storage-drives-media?gacd=9694607-1010-5761040-266790354-0&dgc=st&ds_rl=1285903&gclid=Cj0KCQjw1dGJBhD4ARIsANb6OdknQncauK6iOHaFZj9Mh3bM55chhRpXmBssN5YB4Q9ov2NOj9T4RXIaApFjEALw_wcB&gclsrc=aw.ds&nclid=qsPuhIrAE5BrEOIBLRXrHWDRm0DSSeVfY5urjZC4CvyRO02yd9Se4Y_Y0u6Yg-2t 

Just so you know, I'm neither recommending or not recommending the data splitter cable as I haven't tried it myself, nor seen feedback in this forum on it.

You'd likely also need a SATA power splitter.  Those I have used and use myself.

https://www.ebay.com/sch/i.html?_nkw=sata+power+splitter&_trksid=p2380057.m4084.l1313 

And redxps630 is right - only 2 sata data ports on MB.

@SmokyGutGouger 

8 gigs is not officially supported but does work.  You may need to update to the latest bios before 2 gig dimms are recognized if your bios is very old. Its extremely slow DDR2 and you cannot recover the shared ram for the 512 megs INTEL VIDEO.

https://www.amazon.com/dp/B00D6Q5BJA

You would be much wiser to buy a newer model that is 5 to 10x faster and will work for at least 5 to 10 years more without issue.

https://www.ebay.com/itm/313627194005

https://www.ebay.com/itm/124496608096

"I'm going to upgrade an old 755 for the practice and fun of it."

755, except the USFF, "officially" supports 8GB RAM.  From the 755 tech guide:

I'd never buy a dirty and scratched up Precision T1700.  It would have to be in much better condition.

If a PC is claimed to be 5-10x faster than a 755 or any other model, it would be nice to know for what applications.  We don't know yet what @SmokyGutGouger wants to use the 755 for.

My 755 had an easy time with music recording and editing, but I probably would not recommend for video editing/composing.  Just giving an idea what range it's in.

@bradthetechnut 

@SmokyGutGouger  already stated "Optiplex 755 SFF with windows 10 64bit, so I'm going with 8GB ram (2 x 4)"  meaning 2 X  4 gig dimms which is not officially supported. Not supported does not mean not working but it does mean VERY specific Ram chips.

By the time you add 8 gigs or more of ram and a quad core cpu thats $50 or more you could have purchased a newer system thats much faster with DDR3 or DDR4 ram and will be good for 5 to 10 years more.

The reason I say 8 gigs is not officially supported is due to the original release spec . Official Dell repair documentation also notes this but this kind of information is not available to end users. USFF only having 2x  2 gig dimm slots AND the memory needing to be LOW Density NON ECC NON Buffered ram.  Most users think if its DDR2 and fits in the socket it works without issue. I can tell you that this is not the case. So to say ALL 755 support 8 gigs is problematic.  Early versions of the bios for GX620 GX745 GX755 GX760 do not recognize dual rank dimms or dimms larger than 1 GIG until the bios is updated. Crucial No longer carries DDR2 anything AFAIKT.

DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64 •  Part #: CT722584
 

There were also specific 4 gig dimms (not officially supported)

 PSD24G8002 SDRAM DDR2 800 (PC2 6400)

https://www.ebay.com/itm/265232851241

That allowed 16 gigs with 4 slots minus the 512 meg intel video.

@SmokyGutGouger 

Before updating CPU or RAM update the bios with XP or DOS.

The other gotcha is that you cannot update from A00 to A22 in a single step.

BIOS VERSIONS DOWNLOAD LINKS

o755-a01.exe

o755-a03.exe
o755-a04.exe
o755-a08.exe
o755-a09.exe
o755-a10.exe
o755-a11.exe
o755-a12.exe
o755-a13.exe
o755-a14.exe
o755-a15.exe
o755-a16.exe
o755-a17.exe
o755-a19.exe
o755-a20.exe

O755-A22.exe

In replying some to speedstep post, I forgot about @SmokyGutGouger mentioning 2x4GB RAM.  Speedstep is right.  It has to be 4x2GB for total of 8GB RAM.  RAM configurations are in the Tech Guide also right below where I took that screengrab.

Regarding upgrading the bootable drive to an SSD. Would it be best to leave the HDD in the SATA slot where it boots from now and plug the SSD in the next SATA slot, and change where it boots from in BIOS, OR move the HDD to the next slot and plug the SSD into the SATA slot where it currently boots from? Of course, I have to clone the HDD to the SSD first. Heck, I've never even done that. When I built this PC, I just installed a fresh version of Windows 10 onto the M.2 drive. As far as I can remember, there weren't any special steps, due to it being the boot drive. I did a factory reset on the PC I'm upgrading, so there's nothing on the hard drive I want to keep, except Windows. I appreciate all the advice, thanks.