Software multiboxers use 64-bit Windows versions by preference so we can get that nice yummy 8GB of RAM - which you generally need to run five clients smoothly. Keyclone would be rather useless if it couldn't.

Prefered config is something like a Q6600, 8GB of memory (thus 64-bit Vista or XP), a flash SSD for data dir symlinking, and a relatively decent video card (a 4870 would be nice, plenty still use something like an 8800GT, very resolution dependent as well).