It has an Intel DG965RY motherboard with a Core 2 Duo processor and DDR2 RAM. It used to have a 9400GT but it's busted now, thanks to power surges. I was thinking of a HD 4650 or an HD 5450 to replace it.
I'm not to tech-oriented but my friend says my PSU has a '12V single rail powering 17A'. I'm not sure what that means, but I think it should help you deciding if the cards I mentioned would work.
I'm not sure about the wattage it provides but since a 9400GT requires 300W, I guess it's safe to assume that my PSU has 300W or above.