You could run an 8800 card with that psu (8800GTS). I agree with Codex about Nvidia but the OP has their mind set on ati.
Two things to consider when running an ati card on a Dell:
1.Dell chipsets are either Intel or Nvidia
2.Older Dell systems sometimes have compatibility issues with the X1000's
Well the deal is this. Currently I have 1600 Pro 515 in there. Works excellent. When I got the 8600GT almost had a meltdown as my temp jumped to 113 C. So I just feel safer with ati's.
This card is probably going to be for one more year till I upgrade the system
Sounds like a driver issue. When going from Ati to Nvidia(or visa-versa) you have to completely un-install the old drivers and should use a driver cleaner program. If anything Ati cards run hotter and use more power than Nvidia.
Well, if you do decide to go nVidia, I would definitely go with an EVGA over any other distributor. They seem to be bar none the best when it comes to their warranty and "step-up" program. Sounds worth the while, if you ask me.