Gonna answer 2 in 1 this time:
What modules would you guys recommend for this system? 333 or 400? Any particular company over a different one?
DDR400 would be the best choice vs DDR333.
Memory Support Test @ Tom's Hardware
So, I've been ignoring the A64 craze up till now. Do these require registered RAM?
Standard unbuffered non-ECC or ECC ram works. From what I have read it does not work with registered memory. I also believe its recommended (from Newegg product desc) to only run 2 sticks of pc3200 while it runs 3 sticks of pc2700 fine.
Information on the ABIT KV8 (this taken from Crucial)
184-pin DIMM Banking: 3 (3 banks of 1)
Chipset: VIA K8T800
DDR SDRAM Frequencies: PC1600, PC2100, PC2700, and PC3200
Error Detection Support: ECC and non-ECC
Max Unbuffered DDR SDRAM: 2048MB
Module Types Supported: Unbuffered only
Supported DRAM Types: DDR SDRAM only