3-5 Boxing on a single box.

AMD Athlon Phenom 9600 (2.3ghz)
4GB Ram
3 SATA Drives, however OS is on one, WoW on the 2nd, and the 3rd is for FRAPS dumps.
Nvidia 8800GT w/512MB Ram
Windows Vista

I can run 5 copies without problems, Shattrah can be laggy at times. I set graphics settings to low on 4 slave accounts, high on the main client.

Without adjusting the details (high on everyone) I can easily run 4 clients, but sometimes the drive will start to spin as memory pages out and the system obviously slows down a lot there.