I'm guessing you mean Optiplex 755 DT. Update the BIOS from Dell Support. Just download the file and run it in Windows.That can run DDR2 800 low density RAM. Up to 8GB. The PSU is good up to a GTX1050Ti.
Here are the GPU options for that. It's designed to run a low profile single slot card. In low profile the GTX1050 series are all 2 slots wide with a hard mounted DVI port in the 2nd slot. So that puts you back to the GTX750 series low profile with a cabled VGA port that can be left off or mounted behind the card, or GT1030 which is not as powerful.
If you want to get freaky there is a full height GPU riser for those, Dell# G5459 (be sure and get the card and bracket assy.)
https://www.ebay.com/itm/Dell-Dual-PCI-Riser-Card-G5459-w-Cage-Dell-Optiplex-PCI-e-x16-PCI-x1-Upgrade/153308741888?epid=598516169&hash=item23b1e9c100:g:idsAAOSwpLVcGQqd
Shop around I've seen NOS for the same price.
The ZOTAC GTX 1050 Mini can fit if you trim the plastic cover back to clear the printer port. I would put an Antec expansion slot blower in the top slot because the riser blocks the air outlets that were there before. I haven't tested this setup.
Maximum CPU is the Q9650, Q9550 is a popular budget option. For single thread apps. an E7500 can be tape modded to 3.66GHz.
With 4GB RAM, or a 32 bit OS run a 1GB GPU. The 750 can be found with that capacity.