Another thing to look at if youre using an ATi card is to be sure that in the in-game video settings you have "Vertical Synch" unchecked. Checked or unchecked dictates whether my rig can run 5 toons or not. Also I've noticed server lag (or lack thereof) from different realms can be the difference of seamless or choppy. My experience so far unfortunately doesnt include Dalaran, but I've been (keyclone) 5 boxing on 2 monitors from a 512MB Radeon 4850 / Q6600 CPU / 4GB PC6400.

also, +1 on using a page file ^^