Try using GameBooster http://www.iobit.com/gamebooster.html
I had a friend that had the same problem with choppy gameplay and this program fixed everything. Its worth a try... It stops all other programs that are running on the side while you are playing and concentrates all the PC's "firepower" at "WoW".
Which, unless he has an anti virus running a check, 6gb of ram and a quad should be fine without Gamebooster.