I am not a computer guru by any means so I will simply state my own experiences:

I also have a 30" monitor that runs one client, all settings maxed, perfectly fine, even in Shattrath but if I go to 2 clients windowed, it gets really funky. I do however run with two video cards dual linked to this monitor. As for adding an additional monitor, I have attempted this by turning the dual link off with again bad results. I finally decided on running all 5 clients on the 30" monitor and have had very little problems. I lag a bit when I first come in to town but if I give it time to catch up it's fine from there on. However, my crew has not been to Shattrath yet so we will see.