I'd probably go with an i7 920 for the processor.
You can overclock that a fair bit, with a half decent cpu cooler.
And 5-boxed, wow won't push the processor, even without any overclocking.

A GTX 275 or 285, depending on what you want to spend.
Would be much better then a GTS 240.
And if you upgrade the video, but downgrade the processor, you'll either have cash in hand or another upgrade.

I'll second the SSD recommendation.
Something with decent read speed, purely for the wow folder.
32gb is plenty of space.

If you build the system yourself, you'll likely save $500 over having Alienware build it.
You can buy the parts from Newegg.
There are online tutorials on how to assemble things.
Or find a friend who will help you, you supply the beer and pizza.