I set the maxfps on my alts to 15 frames per second, and it made a very noticeable increase in performance. My CPU usage dropped to around 75% with 4 instances on Outland. I'll see how comfortabel I can 5 box later on today. I'll look into the symlinked copies, but right now the performance is fine. I'm still looking to upgrade for WotlK.

I don't think I have any cooling issues. I'm using an Antec 900 case with like an 8" fan on top, 2 5" fans in the front, one on the side and one or two on the back.