Dell uses a pretty good quality Delta manufactured power supply for their systems. The 4670 uses 47w, the 9600GT 512 uses 61w, 4830 uses 85w, the 4850 uses 110w. http://forums.atomicmpc.com.au/index.php?showtopic=264
I'd have no problem using the 4830 if it were my system. As with all prebuilts that don't have very good cooling, keep an eye on your temperatures. Download a hardware monitor, check temps before you add the card and monitor them after you put the card in.