I am getting my P4 system very soon, and to answer your questions.
In terms of memory, one thing I can tell you definately is stay away from SDRAM. As for the other two memory types, DDR SDRAM and RDRAM are very close in price, though the boards that support them still have a slight difference in price. The RDRAM boards tend to cost more than the DDR SDRAM boards. I would suggest getting RDRAM for a P4 system especially if you get a NW. Their larger L2 cache will take advantage of the higher bandwidth provided by RDRAM. If you are on a budget, then get DDR SDRAM, should still provide a lot of performance, but for all out P4 performance RDRAM is still the only way to go.
As for the Sockets, if you plan on upgrading your P4 system in the future, and even if you don't, stay away from Socket 423. Socket 423 is being phased out in favor of the newer Socket 478 package and all new P4s will be only produced in a Socket 478 Package.
As for the PSU, you need a P4 compliant PSU. It is ATX, but ATX 2.03. P4 PSUs have a seperate 4-pin 12V line that plugs directly into the board. Putting it in AMD terms, your PSU should also be compatible with the new 760MPX motherboards, since I noticed they too use a 12V line that plug directly into the motherboard.
Good luck in building your system.