I dont understand the claims that dual cores cant take 5 WoW's I'm running an AMD Athlon 64 X2 5400+ and it handles everything fine, I never go over 80% usage.

But back on topic, I say suck it up nancy boy. The first 20 levels seriously make you want to play SWG instead (I assure you this is a bad thing) But once you ding 20 or run your first successful instance things get much better.

you might want to play with the CPU affinity and close any unnecessary programs in the background. This should make things run a bit smoother.
You may just want to go to a quad-core which could solve your probs, but why spend money if you don't have to?

Also, some people say that running multiple instances of Wow from one directory burns system resources, so you might want to try making a WoW folder for each account. This may help.