Not sure if this is going to help you but my alt comp runs 4 boxes, my main just one. Alt comp is pretty crappy but runs 4 boxes with max settings.

AMD 4800+ (2.4GHz Windsor core)
BFG 7950GT OC 256MB GDDR3
AN52 (Motherboard)
3GB PC2-6400 5-5-5-18-23
80GBx2 WD Raid-0
Windows XP Pro

IMO and from the looks of it, you really dont need a high end comp to multibox.

From experience I like have 2 comps, lets you run something on main comp and do something like watch movie on different comp with (my setup 2 keyboard but you can use one and do the same.) You can be alt tabbed and keyclone still sends the messages to the other wow windows.