I though about this before as well, and whatever I searched for was not that great and way overpriced.

Something that I have been observing is the trend on IGP (Integrated Graphics Processor) as inexpensive alternatives for discreet/hardware multiboxing. Take into account that WoW, even with the recent upgrades, is not intensive when compared to some of the newer titles out there.
Now check out this review and the gaming performance:
The IGP Chronicles Part 3: NVIDIA's GeForce 9300

The 9400's are just around the block supposedly. throw in a E2200 Intel CPU and 2 GB of DDR2 800MHz and it should be reasonably fast. Also for a system like this a 400 Watt Power supply should be more then enough.
When it is all said and done each "alt" PC should be no more then $500.

Ok, enough theorycrafting