Even getting the 500MHz RAM, you would be running it at 400MHz anyways, unless you plan on overclocking your FSB to 1000MHz!!!
Dual Channel is definatly the way to go with a P4 setup.
XP2200+, Abit NF7, 1GB Dual-Channel DDR333, ATI R9800PRO 128MB, SB Live! 5.1 Digital, TT PurePower 420W, Samsung 120GB, Maxtor 120GB, LG DVD+-R/RW, WIN2K PRO SP4