Well, I just tested this out on my personal computer. I currently have a.......

Intel Duo 2.20 with 3.25 gigs ram (Would be 4 gigs if I had Win XP 64bit) and a 8600 GTS 512 MB Video Card,

I ran 5 accounts ALL in Stormwind (busier) and ran around with them all "Autorun" my computer could run it, but it was a little choppy, and I will not be 5 boxxing on this computer with out a back up to handle 2 accounts. Which I could do if I setup my g/fs computer next to me. But this was just to help give you an idea.

Also I'm running witha 19' as my main with 1 account, and the other 4 accounts spread on a tiny 15'' moniter next to it lol.