dontlistentome :
I'd consider something like an AMD HD 7750 if you can buy used. It still gets driver support and for a Core 2 Duo system it should be a nice fit. In new cards you could go with a GT 730 DDR5, which I usually don't recommend but you have an E8400 so it's not a bad match. Or you could splurge on a new GT 1030 but make sure it's the DDR5 version. Check prices and go with the least expensive.
I assume you realize you will have to focus on dual core friendly games. No GTA V/Assassin's Creed Origins/Far Cry 5 on this system. At some point add a cheap 120gb SSD for your boot drive. It won't help game performance, but it will make the computer feel snappier overall. If all you care about is game framerate then an SSD is not worth it.
Thank you for the quick reply. Long story short I decided to go with the GT 1030. There were different companys that sold it and they all looked different. But I chose this one its not low budget but I figured what the heck ill spend my money's worth
https://www.amazon.com/gp/product/B07646VQ6T/ref=od_aui_detailpages00?ie=UTF8&psc=1
I double checked the specs to make sure the one I'm buying is right for my optiplex.
So far so great it is PCIE x 16 like I asked and is DDR5 SSDRAM. I'm not sure what the SSDRAM means but I hope its DDR5. This is my first time changing my PC'S GPU. I'm hoping everything goes well and ill let you guys know what happens. Also I am assuming because the GPU I brought is PCEI x 16 it takes 75W? like I asked. Also I'm hoping the GPU fits and is not too big. Like I said this is my first time doing any of this stuff. But I worked on other old Computers on taking them apart and reassembling them back together to gain some experience and find what I'm working with.
And I also invested in memory RAM
https://www.amazon.com/gp/product/B0050D6KXI/ref=od_aui_detailpages00?ie=UTF8&psc=1
I also double checked the specs to make sure its for my optiplex 755 and it is. I brought two of them because the ones that are in stock right now in my Optiplex are 1GB each and all four RAM slots are used by the 1GB's. So I'm replacing the 2 1GB's RAM slots with 2 2GB's RAM - 4GB total. I'm Hoping everything goes well.
Also I saw some other GPU's came with installation disks and the one I brought only came with a manual and no installation disk. Is this going to be a problem or am I manually going to have to look up the GPU info and download the software for the GPU?
Thank you for the help and replies!!