Wow almost does not notice the difference between a 50 dollar throw in video card and a top of the line triple crossfire or sli system.
This might very well change with Catacylsm, supposedly the games engine will actually use the card for more than rudimatary processing.
In my experience for WoW a multicore CPU, dual core minimum, of at least 2.5ghz is the very first thing you should consider to optimize your WoW experience. Right from the mouth of one of the games developers, though I don't remember which one, WoW runs two big threads and sometimes two or three little ones.
WoW is also a ram whore. I currently have 8GB of Ram, casual observation,has shown WoW to shoot up over 3GB of usage occasionally.
I have a 9800gtx (I am probably upgrading to one of those gtx460s sometime this year, the time feels right). I have ever setting at max except shadows. Keep that at minimum no matter what rig you have, until Catacylsm, hopefully the engine upgrade will make that setting work. On my system prior to this, AMD64 3200,6600 GT w 2GB I also ran at max settings, albeit at a smaller resolution (I play windowed on my main monitor and have no problem watching hulu, netflix or a dvd on the secondary monitor).
The number one thing that would improve my WoW performance, which I am trying to save up for, is a SSD.
If I were you, I would try to work 8GB of ram, and a SSD to boot from and run Win7 and WoW from. Alternatively, run WoW from its own hard drive. That gives a slight performance boost. WoW is notorious for tons of small file accesses on a continual basis.