Actually you can use a 400Mhz FSB chip with dual-channel memory:
400Mhz FSB - Dual Channel PC2100
533Mhz FSB - Dual Channel PC2700
800Mhz FSB - DUal Channel PC3200
However, unless you are on a real tight budget just get a Northwood C CPU - 800Mhz HT
Even if your budget wont stretch that highm wait a month or so until it does

Get some TwinMOS DDR400 memory (usually nice and cheap and good quality) and you'll be laughing.