I have several Optiplex 760's and have worked and played heavily with 755's and 745's. You could probably get decent gaming performance for the games you are playing out of a Radeon 7750.http://www.newegg.com/Product/Product.aspx?Item=N82E16814150612
It is single slot so the BTX orientation of the Optiplex won't be a problem, and it will run off the 75 watts provided by the pci-e slot.
You won't get monster frame rates, but at low resolutions and low textures it should be able to play nearly everything on it (BF3, Diablo3, Skyrim, Crisis, etc). Less demanding games will run much better obviously.
And I disagree with Sinister. They don't "Generate a ton of heat". They are very efficient because of the BTX layout and the power efficient cpu (65 watts). I've found the power supplies to be reliable and of good quality. Plus these machines are ridiculously inexpensive these days.