More than 4gb of ram will not improve performance in WoW.

World of Warcraft is a 32 bit application, and is therefore restricted to the good old 4gb cap. And like any good party, all the people want to cram into the same room; the same is true for ram utilization: Vista, warcraft, ventrillo, Firefox, what have you, all want to utilize the lowest reaches of your available ram, cramming it full.

Trust me, I have 8gb of ram, 512mb video, and my ram utilization has never topped 3.7gb yet Windows keeps using a pagefile. ;(