What operating system are yeh running.

With xp you only get about at max 3.5 gigs to use, xp also require at min 1 gig for itself to run.
So with using xp with minimum of services running and no other apps you have 2.5 gigs for apps.
Wow require anywhere from 300 megs to over 800 depending on resolution yeh play at, to where in the game yeh are.

The only time yeh should have yer harddrive pegged is when yer loading into a new area/instance and when yeh dont have enough memory and it'll try pageing out one of the wow's which will cause it to time out and log off.

vista require almost twice the amount of memory to run as compared to xp.

Your not gonna get any improvement using another folder/drive