MontyB+MightyMacbeth
Almost all MB have 4 DIMM slots now, as long as he buys a namebrand ASUS, MSI, GIGABYTE, DFI, he should be OK. The 2-3 slots are being phased out because the maximum amount of memory supported is increasing. As for memory he shouldn't get 512 Sims. Thats a waste. He should get 2x2048 or 4x1024. Keep in mind that Windows will only recognize up to 4 GIGS, so anything above that is a waste.
As for the video card, I grant you that the 7 series is the more stable card (although that is not to say the Series 8 is unstable). But given the fact that you will have to upgrade to series 8 at some point soon anyway, it seems like a waste of money to buy the old tech now. I simply don"t see the reason of buying a 7900GS now and then buying a 8800GTS in 3-6 months. Nvidia isnt like ATI, you cannot mix two different chipsets. Its 200€ wasted.
As for PSU, at least 500W. I have 600W for my pair of 7800GTX. Make sure its a good brand too Enermax, Hiper, Antec, Coolermaster, Asus.
Concerning the CPU.
Due to the AMD+ATI merger, it would be very unwise I buy a Intel CPU and a ATI card. They are competitors and although Intel has said will make a chipset for crossfire, the bulk of their concentration will be for Nvidia.
MightMacbeth
Monty and I have a disagreement about which card you should buy.
I personally think you should buy a series 8 (8800GTS) but thats my opinion. Of the 3 cards you mentioned. The 7300 is a budget card, it'll do games but not very well. I have one on my server. The 7900GS is a vast improvement and is more oriented toward gamers. The 7950GX is faster that the 7900GS but not by too much. Since you'll probably be ditching this card anyway I would get the middle-of-the-road range. BTW Nvidia as stated that they will have a 8800 budget gamers cards sometime Q1 2007. You might want to get that instead if the price is the same.