I know virtually nothing about graphics cards, other than that mine is inadequate for D3. I really would not like to spend very much money, since the only high-graphics games I play on my PC are Starcraft 2 and Diablo 3. Currently I have an ATI Radeon HD 4200
I think you might be able to put a Radeon HD 7750 in there. I know that card uses extremely low power and you might be able to get away with not getting a power supply upgrade. I can't find any information about your PSU so I can't suggest anything better.
The 7750 is a good card, so you could probably play Diablo 3 on Medium to High settings depending on your screen resolution
Yep that's what I was going for *hopefully* it's enough to run a 7750.
if its at least 300w, he might feel a bit safe because i would assume the 12v rail to have ~19amps of power, and the 7750 taking less than 9+rest of computers requirements should be fine. if its under 300, then there will probably be a problem somewhere.