well hes got:
Intel Pentium 4 2.8GHz
512MB DDR2 RAM (2x256M)
1x Dell Manufactured DVD-RW (no idea what model)
I would say simply get the GeForce 6600. The 6600GT will be pushing it.
The GeForce 6800 is possible because it actually uses less power than the 6600GT. A little risky but safer than going with the 6600GT.
See below for a long winded answer.
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Okay here the deal. A typical for the typcal 240w PSU, the 12v rail will have 16 amps, for a total of 192w. The 12v rail powers the CPU, GPU, and splits the load for the hard drive/DVD-ROM with the 5v rail.
A Pentium 4 3.2GHz CPU uses 144w. I'm guessing a Pentium 4 2.8GHz uses around 124w.
The 6600GT GPU uses 48w.
The general concensus for the typical hard drive and DVD burner is 15w each or a total of 30w. But I've read that the load is split with the 5v rail. How much? I don't know. Say 67% is on the 12v rail so that's another 20w.
Total estimated load on the 12v rail is 192w which is the probably the same as what is delivered by the PSU (12 volts x 16 amps = 192 watts).
The good news is that is assuming the CPU, GPU, 1 hard drive, and 1 DVD Drive are being totally maxed out at the same. It is unlikely, but technically speaking it is still possible.
The GeForce 6800 (not the GS, not the GT, but plain 'ol 6800) uses 39w of power. So that brings it down to a max of 183 watts. Still pretty high with a slim margin of safety, but actual power consumed will be less since I will be unlikely he will be burning a DVD while playing a game. Also it's not like as soon as the game the CPU and GPU are both stressed at 100% along with the hard drive. So the actual max might be something at 170w.
While these numbers look nice on your screen, it's just a bunch of numbers thrown together and real life tends to throw you curve balls.
So get the 6600 which uses 28 watts of power or get the 6800 which uses 39 watts of power. Just remember one thing: Do not overclock the video card, to do so will cause it to draw more power.