I had that system for a while and there's a few things you need to consider. open your case and check how much room you have, because it won't be much. check your PSU (probably 300 watts) and see if a card can run on that.
I think the 7600gt that osprey linked would be a good choice.
that dell thread made me laugh. I bought an 8800 and was having the same problem. I decided it wasn't worth the hassle to dremel half my case apart, so i bought a new mobo, case, psu and threw everything in it.