The hardware is overkill for 5 instance of wow. The problem being vista increases the memory requirement quite a bit.
Vista need around 2 gigs of memory without paging anything out to the hard drive each instance of wow needs anywhere from 200-500 megs
once yeh get to an area where yeh need more memory then yeh have available stuff gets paged out to the hard drive which causes lag. Which could cause anything from loosing follow on one char to as bad as watching a slide show depends on a variety of things.

If yeh cant get vista64 I would stick with XP.
Yeh can make vista32 work just fine but yeh need to tweak it and wow some.
If yeh get vista64 get 8 gigs of memory.