first of all your processor is plenty fast enough to run 5 instances of wow. you could run 10 with that processor if you had the components to support that many.
I think your easiest fix would be to upgrade to the 64bit vista and add some ram, like you already stated, but that might not fix all your problems.

running that many instances on one machine creates two other weak points, your hard drive and video card. you made no mention of your hard drive.

what is your video set-up, 1 or 2 monitors and what size? also check to see what programs are running in the background and turn off everything that is not necessary to run wow. this could free up enough memory to speed the game up enough to make it playable for you.

try turning down the settings on your slave toons. turn their settings down the absolute min. I would try some free solutions before I paid for an OS upgrade.