If you are looking for speed increase just for wow - wow only takes like 12 gigs or so. So no need for more space. If you want to store other things on that drive, it might make sense to get something bigger.

But keep in mind, 2x 72gig drives are going to be a lot faster than 1x 500g drive as far as running multiple copies of wow goes (and probably anything else).

Right now, having 6 gigs of ram is going to a be your bottleneck, assuming you run 5 copies of wow. Each copy of wow takes close to 1gig memory, plus the OS can cache used with excess memory. in addition, the OS overhead of Vista is close to a gig, not to mention all the other utilities and programs you have on the machine taking up memory. I have 8 gigs of ram and run 4 copies of wow and I'm maxing my memory out and it's accessing page files on the hard drive.

So I think you're better off getting 12 gigs of ram vs another hard drive if you want to maximize wow performance.