You could use i7 1st gen or higher, and 16-24 gigs ram, a nvidia 580+ video card or higher. SSD drive, enough to hold windows 7 and wow - probably 128 gigs min space.

You'd need 2 of these, each can run 12 copies of wow. Total 24. That's probably the cheapest way to go.

If you wanted 6 different machines, I don't know what you'd use. Maybe a q6600 and 8 gigs ram or something, and a nvidia 400ish series video card.

You want to stay away from any VMs - they aren't going to be able to handle the graphics, and it would be a PITA to switch to 24 VMs to configure anything. Normally hosting companies aren't going to have any graphics card besides a 16 meg VGA on-board adapter.

You could always buy a machine and try to co-locate it at a hosting center, but terminal servicing into it would be horribly slow, and it would be probably frustrating as hell trying to configure anything in wow.