I got a pentium4 with XP, 2 gigs running 4 accounts. What important is what else yeh got running?
Yeh can have the fastest box money can buy and it still wont run 5 accounts if yeh got a bunch of junk running in the background.
Once yeh have yer box I would suggest doing a lil research on optimizing the operating system for speed.

I would suggest getting 4 gigs of ram if yer getting XP and 8 gigs if yer gonna go with vista.

You could probaly run 3-4 instance of wow per core.