First get a 64bits OS and more ram (1GB per client + 1GB for the system as a rule of thumb to be really on the safe side).
If this is not enough, get an SSD and install you OS and WoW folder on it.

This two things made a wold of difference for me. I used to be unable to do anything with more than one (max two) characters logged in Dalaran, now I can ride around with Crusader's Aura without issue.