Basically, if you go LGA 775 now (800MHz FSB), it will cost you more and you probably won't get much of a performance improvement, if any over a decent socket 478 setup. Don't be fooled into thinking DDR2 is some awesome jump over standard DDR modules, the latency is through the roof, which isn't good. Personally I'd go for a setup similar to mine, see sig
The OCZ RAM I have is low latency and capable of running at DDR500 and higher. To get the best out of any system, you should match the FSB of your CPU with your RAM clock speed to get a 1:1 ratio. That's why I got the PC4000 modules so when I overclock my Prescott, I still have a 1:1 ratio, I currently have them synced at 240MHz FSB (x4 = 960MHz).
Plus PCI-Express is going through a teething phase at the minute, it's new tech' and in a lot of cases is out performed by AGPx8. Another downside to LGA 775 is the fact that Intel decided it would be a good idea to place the pins on the motherboard instead of on the CPU, so be very careful. Don't forget the 875P is a mature chipset, it's a tried and true architecture with brilliant performance and overclocking potential. Abit IC7-MAX3 or Asus P4C800-E Deluxe are the best 875P boards on the market.
As for the PSU, I'd recommend Antec or Enermax around 500w