5-Boxing with IS Boxer on a single PC:

i7 920 (stock)
Asus P6T (original)
12GB DDR 1600mhz
GTX 275 1GB.
Patroit SSD, 220mb/sec sequential read.
Vista Home Premium, 64-bit.

On medium settings (maximum view distance on the main), the system is fine everywhere.
I've occasionally noticed a little latency in very crowded areas, but I can Army of the Dead with 30 horde inside Galv's room and run about killing them without issues.
Stormwind, near either AH on a Saturday afternoon is a lot more of a strain on the system.