Just because you aren't at 99% cpu and ram used, doesn't mean you aren't being limited by them. Efficienty comes into play. I have 8gb of ram, and each of my wow windows uses upwrds of 500mb, and i get great performance (as long as I keep em all on the same video card's monitors, diferent issue there) I turned the graphics all the way down on the slaves, and capped them at 60 (they would each run up to 80-90 fps before) My main is also capped at 60, but with the graphics all the way up, and at a higher resolution, sometimes dips to 40fps, but that's still perfectly fine.

I regularly see my CPUs at 85-95% utlilization accross all 4 cores, spread pretty evenly, while running 6 wows. I'm pretty sure that I am only limited by the CPU speed, available RAM (would love to go to 16gb, just to see if each wow takes up more of it, and the fact that ram is so cheap) and my bus speed. I have a Velociraptor for my main wow, and a regular folder for all of the slaves. I used to run 5 separate folders, but it got tiresome, and when the 3.0.2 patch came out, I didn't want to bother copying 4x, so i moved back to one.

My next upgrade is going to be a motherboard with 1 full PCIe x16 slots, (my second one is x4 electrical) with 16gb of ram, and whatever generation of quad processor is available (unless the 6-8 core ones come out soon). I also plan on using SSDs either for the whole OS and wow, or just for wow, and stick with velociraptor for the basic OS. A stripe of 4 32gb SSDs sounds juciy and ill just back it up to a 160gb SATA hdd every night