Here's a spec that works for 5 boxing on 1 machine using Keyclone:

EVGA 680i MB

Intel Core 2 Quad Q6600

4gb DDR2 800mhz

Single EVGA 8800gts 640mb

Using 2 monitors, 22" @ 1680x1050 & 19" @ 1440x900 running my main on the 22" and the slaves on the 19". Main set to 60fps max and the slaves at 30fps max