
512MB is plenty for almost all users, no need for more. Outside of Dual Channel DDR a P4 really needs PC2700 bare minimum, PC3200 is much more preferable. If you go DC-DDR then Granite Bay is tops BUT hugely over-priced, SiS' DC-DDR solution is a little slower but still offers a nice perf boost. Problem with going P4 is that you tend to pay a LOT more for no perf gains. If you go P4 2.66ghz is a very good choice as stock perf is great, it o/c's very well and doesn't cost the Earth.

If you go AthlonXP you can get a GREAT bargain from TbredB XP2100+ (ensure it is TbredB though) as these often hit XP2700+ speeds with 333-400FSB (mobo and RAM permitting). If you want to guarantee TbredB then XP2400+ is what you want and tend to averagely o/c a little higher. Barton shouldn't be overlooked though, it doesn't give a huge boost (5%ish clock for clock) but in XP2500+ (always Barton core) you're guaranteed 333FSB from the off. You'll want a good HSF for these though, Volcano 7+, Volcano 9 and CoolerMaster Xdream are all good coolers which cost very little. As for mobo choices nForce2 is best but unless you want every last 1% of perf and the great onboard goodies it is definitely worth at least looking to KT400 or SiS746FX, SktA really doesn't need DC-DDR (2 RAM sticks req'd of course) although with it nForce2 does offer a 5%ish lead in speed over the 2 key alternatives. KT400A is due out soon and despite not having DC-DDR it yields nForce2 with DC-DDR perf. It will use 8235 at first (same as KT400) but 8237 should soon follow and upgrades the sound to much nearer nForce quality. KT400A isn't out yet so we only know from pre-production reference designs. Finally RAM on the AthlonXP is much easier, PC2100 is fine so long as you're happy at 266FSB but PC2700 is the minimum sensible choice and in truth nothing more is truly needed, PC3200 is worth it to either eek out every last 1% from your nForce2 (only chipset which often achieves 400FSB) or simply run as PC2700 with fast timings and enjoy the better future proofing.

P4 tend to cost you more but run a little cooler and quieter, it is pretty even and using the XP rating vs the P4's speed is a very good guide to perf.