Just for the record, I run 4 instances of WoW on an E2180 "Pentium Dual-Core 2.0GHz" CPU OC'd to 2.8GHz, 2GB of RAM, with a GeForce 6600LE card in a crappy Shuttle. This is truly a slow video card by today's standards since it doesn't have a fan it is underclocked compared to a typical 6600. I only get around 15fps on each of 4 windows, but for the secondary box this acceptable. I'll probably upgrade this machine to an 8800GT in the near future, but my point is that the CPU and RAM are not extraordinary and I wouldn't recommend dropping a ton of cash on overpowering a secondary box.

I know it's not relevant to the discussion, but in comparison my main machine is a Q6700@3.2GHz, 8800GT, 8GB of RAM although I can only use 3.4GB since I'm on XP32.