I had general performance problems at first. I found three things made a difference (although I can't say which was the most effective):

In order of cost:
* I turned down graphical details etc in WoW
* I bought a £10 4GB memory stick to use Vista's ReadyBoost feature
* I bought a solid-state drive, and copied WoW to it

Previously, if I took all my toons to Stormwind, I'd have huge pauses of up to 30 seconds at a time, making it practically unplayable. I'd board a ship, and before all 5 copies had finished loading the new continent, the ship would be on it's way *out* of the destination harbour...

Now, maybe one of my slave windows gets a bit jerky if the place is crowded. The main window remains smooth as silk. The destination continent is loaded almost immediately when using boats. It's a vastly more pleasant experience now!