I am working on a Dell Vostro 200 right now. I added an Nvidia 8600GT into it without issue. It's been running perfect for over a year. these cards are well under $100 now.
I was a bit worried of the PSU not being able to power the card directly, but I haven't had any issue whatsoever, and that is with heavy 3D gaming usage as well.
Good luck,
Anthony