Here is my system.

Phenom X4 black edition2.6ghz, 8gb ram, ATI 4870 HD 1gb, vista 64, wow on a SSD os on a raptor drive. I really dont have any problems 5 boxing with innerspace so far. I play at a 1280 x (whatever here) resolution fine so far up to about 5-6 accounts if not in dalaran its NP.

I think if I were to 10 box I would do it on 2 computers. This way I can just focus on 2 machines (os licenses) hard drive video cards ect would be easier to upgrade then doing 5 pc's. Along with power consumption and deskspace. You can get comparable PC's now for like less then a 1100 bucks I think lol .