Jordan,
I have the same OptiPlex 755 SFF that it appears that you do. If I were you, before I started doing any upgrade to the system, I would make sure the system was working properly first. I did not see a HD in the picture you posted. I would get the system up and running to verify that all is working. Then I would go to the DELL drivers website and download and install the latest BIOS for your system. The latest is A22. That should give you the ability to upgrade to any of the core 2 duos or quads. I do not know anything about the Celerons, as my system came with a E8500 core 2 duo. I think the original build date for mine was in 2009. Anyway, I just finished (30 minutes ago) replacing the E8500 in my machine with a Q9650. That is a core 2 quad running at 3.0ghz. The E8500 was a duo running at 3.16ghz. I did the upgrade for a number of reasons. I wanted a quad core processor which also had twice the L2 cache at 12meg vs 6 meg of the E8500. Anyway, the cpu upgrade went fine and everything is working as it should.
My system also came with the ATI RADEON X2400 PRO 256MB low profile video card. I just purchased an upgraded card that DELL lists as compatible. That card is the VisionTek Radeon HD 7750 SFF 1 GB GDDR 900549. Here is a link to the DELL accessory web page that lists the various accessories for a number of their computers.
http://accessories.us.dell.com/sna/...d=192261&chassisid=8341&tab=Parts&stype=2&p=1
I am not a gammer, but from what I have been able to read, the 755 makes a pretty good low end gaming machine. I use mine as part of a software defined radio system I have. It is not real graphic intensive, but it can eat up resources.
My system is also currently running VISTA 32 business. Along with the video card, I am awaiting arrival of a 64 bit copy of windows 8. I will upgrade it as soon as the software arrives. Also from what I have been reading on a number of forums, there may be some issues with going to windows 8.1 due to some possible hardware issues. The upgrade to win 8 required a more capable graphics card that what is currently in my machine.