I'm using a HP Dv9000 which is around 2.4ghz with 2gb of ram, has a GeForce 7400 in it. I run 5 accounts just fine in instances and running around. The only problem I have is super high population cities like orgrimmar which make me lag so it takes me a few minutes to get past the entrance, other then that it works fine.

The key here is setting Maximizer to have your non-focused windows around 10fps. I can still see what they are doing and its not hogging all the resources trying to update them smoothly. I really don't need to look at them at all since I can see who is casting from my mains add-ons (xperl frames). If I need to play on them, I have their focused FPS around 20-25 which looks just fine in their smaller windows. Of course my mains fps is 15 minimum and around 40 focused cause I want to see the pretties XD