I HAVE NOT TESTED THIS. But I have tested a lot of hardware and consider this to be a ballpark estimation. Don't buy a system without further input but this is a good guide for basic 5 boxing on a single system. For advanced PvP, you would probably want a beefer system. Load times are going to be a pain in the ass no matter what you do, short of going pure RAM based hard drive (which IS possible) or perhaps a solid state drive based system.

If you play with 640 x 480 or 800 x 600, minimum settings and use Keyclone, I would imagine a reasonable mid level dual core with 3 gigs of memory and a 7600 or better nvidia graphics card in horizontal span would be playable in most settings with 5 total WoWs running.

To be on the safe side, go with an 8600 or better (the price difference isn't much and WoW is GPU not CPU intensive). I would imagine such a system would be able to handle 5 WoWs for under $1000 for the entire system. Maybe even under $800.

But get more input - I boxed with single core systems 3 years ago when there were no dual core systems out there. I can play 5+ on my quad core system no problem..... but there are other factors out there too - so YMMV.