Definately an overkill IMHO.

I have a single 920 i7, 6gb ram, 2 vid cards (gtx280) and it all runs more or less flawlessly.
Slaves are all limited to 20fps and in dalaran only does it get strained. Wintergrasp can become pretty rough too but the servers lag out and make it unplayable anyway so *shrug* :P

The thing with WoW too, is performance doesn't really improve if you assign more than one physical CPU core to it. So two checks in the task manager probably is the same as 4 (two phsyical cores) since the game isnt designed to run over multiple cpus. This is assuming no other processes are assigned to those cores.
My core allocation is core 2-3 for main, 4-5 for two slave sessions, 6-7 for other two slave sessions.

Also keep in mind you'll need to change the primary monitor to one of the monitors on each physical card when you open up a new session. If you have 3 cards it's gonna get messy!



Edit - Would recommend 2vid cards and wide screens instead, makes the PC more usable for multimedia stuff / other games etc when you dont have to look at bezel lines everywhere, seeing as you're using keyclone n all.