The PSU in your Dell will probably work perfectly. Before my new XPS420, I had a Dimension 4700 with a 305W psu. I ran a 7600GT, 7900GS, and a 8800GS with no problems. The 8800GS ran kinka hot, but that was due to poor air circulation in the case. In my XPS420 the same 8800GS runs around 15C cooler.