Thats a good spec for a 5-box machine, it'll work fine.

The most important thing with WoW is the Graphics card.

The new 8800GTS is great and well worth buying in my oppinion.

You'll do fine on PC6400 RAM, but if you want to make it PC8500 or so, then do it because you'll notice the diffrence for other stuff :-D

Best of luck.