[NAME]  There isn't any conflicting information on the 755 as far as RAM, CPU, and OS.   Those issues are well discussed and verified for over 14 years now. Please press the blue Accept as Solution button below if this answers your question. Windows 10 drivers do not exist for this model but windows vista or 7 drivers work just fine. Not supported does not mean not working. There are zero issues with an ssd drive in almost any model dell from 2006 on as long as bios is set to AHCI. The default F2 BIOS SATA OPERATION is RAID which will not work because it requires F6 mass storage drivers. Setting to AHCI for all sata drives both DVD and HDD will allow an OEM Windows 10 DVD to boot and install windows with the I don't have a key option. Once these DVD are gone they will never be available again ever. OEM WIN10 DVD $15  
Folks, excuse my ignorance, I just did my first gaming PC build, I'm learning, but I'm still a rookie. I'm going to upgrade an old 755 for the practice and fun of it. I took it apart, literally not two pieces were together, cleaned everything, then put everything back together. lol  So, I'm confident I can do any upgrades that are possible. It's an Optiplex 755 SFF with windows 10 64bit, so I'm going with 8GB ram (2 x 4), an SSD in place of the old HDD. 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. The CPU upgrade is the area I'd appreciate some advice. From what I understand, more CPUs will work than are listed as officially compatible by Dell. If someone could share a link to a list of CPUs that are known to work, beyond the ones listed by Dell, I'd be very appreciative. Any advice is welcome, for that matter. Again, excuse the lack of knowledge and the long post. 
Optiplex 755 CPU compatibility