So, before you go spending a load of cash on multiple systems, look at what I have to say

For example, I 4 box shamans and I am smooth and lag free in instances and in cities, but a little jitter in high pop cities like dalaran, but thats because my computer is getting upgrades in a couple of days to overcome this.. My computer is a Dell Dimension 8400.. old as time..-

Intel(r) Pentium(r) 4 CPU 3.4GHz- (this is being upgraded)- with your quadcore this should not have to be replaced other than the fact that sometimes dual core is better because it does not overclock as much.. which is hard to explain.. I would keep u quad-core though.

4GB of ram- upgrade that improved my computer tremendously from 1GB, this cost me around 50-100 bucks depending on if you get special gaming RAM.

Nvida 6800- (tried out the 7900 of my dads today but adds a little more lag so ewww ).. this is fine becuase i really do not care how well my alts look on the screen.. so w/e.

When I am running these 4 accounts I am lag free and it is fine, but i have the graphics down on alts, and the main has some of the graphics up. So, as you can see my computer is basiclly factory shipped 7 years ago and holds this up. But if your looking for extreme preformance and have a lot of money I would go down the path of mutliple computers