You didn't say what you're using now, so I have no basis for comparison, but my suggestions would be:

> Dual or quad-core processor, running at 2.4GHz or better. Probably cost you $200-300, not counting a motherboard ($100-200).

> 2GB RAM minimum. 3GB recommended. You can get 4GB, but unless you are using a 64-bit operating system you will lose access to about 750MB (at least with 32-bit Microsoft Windows). On the other hand, the prices of memory are very low right now, and it might be worth it to buy four 1GB or two 2GB modules even if Windows wastes some of it.

> Decent video card, like a NVIDIA 8800GTS with 320/640MB memory. Prices starting at $300? Not sure.

For comparison, I 5 box using two systems. The system that runs four of the characters uses an Intel quad core CPU at 2.4GHz, 2GB memory, and a NVIDIA 7800GTX card with 256 or 512MB memory. I run four WOW instances in 800x600 with all graphics options turned down but running tiled on a 1600x1200 screen. They run smoothly pretty much anywhere, 30-60 FPS even in cities.