I've been 5-boxing with a 24" iMac (main machine) and 4 of the march/09 mac Minis (the first model with the GeForce 9400M graphics) since they came out. The Minis work really well for slave boxes, I use them on 19" 1280/1024 monitors with most of the settings down low and they get ~30fps most of the time. I use a wireless keyboard, 5 receivers, and synergy to allow easy mousing around the various machines. There's also a trackball on a KVM for the slave boxes but I rarely use it.

You could get something cheaper if you BYO windows box, but mac minis hold some resale value (and of course can boot windows if you wanted to multibox a win-only game).