Let me just say, from personal experience that is, having two 22" or 24" may be overkill.
Until last night I was happy chugging along with a 22" wide as my main and a 19" as my alt screen. Now the 22"s there ... well I haven't set it up but god damn theres a lot more space now.

As you're getting a quadcore, you can use Keyclone to designate cores to separate copies of WoW, so that each (bar 2) run on their own core. The ram will be enough for the job.
and as long as your graphics card is mid-high end (I'm running 5 on an 8800gts 640mb) you're fine to just have the one card for your setup. Most bottlenecking occurs with hard drive I/O, which is where mklink comes into play (search for it )
If you really do want to you can add a second graphics card for the other WoW's, but theres no real need. If you're going to be running vista keep in mind the cards have to be the same brand. You can no longer run ati and nvidia drivers at the same time, and all cards run off the same single driver.

Have a quick look into Keyclones maximiser (and PiP - picture in picture), as it will be able to handle what you're after.

Good luck and have fun!