You'll need a 64-bit OS to take advantage of any RAM beyond 4GB.
Depending on the number of monitors you use and the resolutions you run WoW in, your video card may become an issue.
And you will also want to be certain to update the Processor Affinity of your WoW copies to take advantage of the four cores.

Aside from that, looks very similar to the setup that I 5-box with.