Graphics card & video card for a Dell Optiplex GX 755 (not the mini tower!)

Ok so I just recently got a Dell optiplex GX 755 PC that has- 

*Intel core 2 quad processor 2.4 GHz

*8GB ram

*320GB hard drive (kinda small i know, i'm looking to upgrade that as well)

I'm posting this here to see if anyone knows mainly what graphics card i could get that would make my PC gaming friendly. It doesn't have to be the most amazing stellar quality card out there since iv had (some) people tell me that you cant upgrade these PCs much. But just some good quality advice on the subject and maybe a recommendation on what card brand to get as well as the specs for it. Also a recommendation for a video card would be greatly appreciated as well. Sorry for my ignorance on on the subject, i am new to the PC world. I ended up having to get this computer in a pinch because I am a small time graphics design artist and my laptop died, and I had orders coming in. I'm not looking for a quick fix to the problem so even if anyone has a solution that may take a bit or quite a few steps to get underway, the help is still greatly welcomed! I am hoping if its at all possible to make a good situation out of this and maybe build this PC to fit my needs. Thank you to anyone who can help :)