use 2 different hd's, one for OS and one for WoW as well helps when there is memory intensive applications/swapping going on and windows will swap alot no matter the amount of memory you have.

Hard set you swapfile to recomended size both min and max, windows has "it's" best intentions in mind not your WoW mboxing, remember that

also look at which multisampling you are using in wow, if you are running your alts at 24bit x4 downgrade to x1, try doing same on main if you get better performance then you need a new gfx card to be able to run at "max"

ohh and also, defrag...