Need to know a lot more than PSU.
What are the rest of the specs on the system? (CPU, RAM, etc.)
What kind of graphics slot do you have? (PCI, PCI-E or AGP)
If that PSU is decent (Dell ones usually are, actually), you could get a 9600GT, which would be loads better than an 8600gt or 9500gt.
Heck, I could sell you mine for $60 shipped (negotiable), not to shamelessly promote myself. I think anyone could agree that it's a fair price, considering it's been used for less than 6 months.