Also, Dell have started to use BTX in a bunch of computers - that's quite a different layout (motherboard as well as case) than standard ATX, and I believe the PSU is also different. But you should be able to buy aftermarket BTX PSUs... dunno if Dell does more customization than that?
Btw, I have a GF8800GT/512meg, 2 x 74gig 10.000rpm raptor drives, a quadcore Q6600 overclocked to 3GHz, and 8 gigs of ram... and my system totals at ~210W under load. You might not actually need more than a 350W PSU after all, as long as it works stable under load