If you want to run 5 on one machine you'll want a lot more than 4GB of ram if you don't want to lag in cities. running on multiple machines is the best route to go if you're able to do it - you can do it without a KVM by using Synergy with broadcasting enabled. You can check the WIKI out here for information on where to get it (Synergy is free) but you will need (i believe) additional Keyclone licenses.

Another suggestion would be to avoid the main cities (SW, Shat, IF) for alliance and sticking to the less used cities. Darnassus or the Exodar (sp?) are a lot more workable and with 2GB on my laptop I'm able to handle those 2.