well il give you 2 tips i used in the past before i multiboxed with the help of a keyboard clone "aid" and is still using.

make the other windows small as hell. the minimum is rather small .
that will lower ram by alot.

BUT the best one is to have the other windows minimised.

im on 2 gb of ram with 5 wow running. and not really having much issues besides pwnboxer bugs :P.

*dang i went afk and forgot what i wanted to say...*