Well, I have a machine slightly better than yours. At least my graphics card is 8800GT. However, it's never been successful for me to run 5 copies of WoW, either in same or different directories, smoothly without 'hiccups.'

Of course, I tried to figure out why. First, I changed all to 800 x 600, no use. Then, I turned sound off on all of them, no improvement. I set all graphics settings to min, low or off, no improvement again. Then guess what, I use 0 addons on any copy and the FPS increased!!!

But hey, how can I use no addon?

Atm, I used 3 to 4 addons on the toons. One is inventory (that combines bags), another is for the actionbar, another is autoaccept invites/quests, and the last one is ug_frame UI.

I plan to remove the bags addon once they reach 70 though: toons wont need to pick up that many things in instances. The autoaccept addon can be removed as well since there wont be many quests to share at that time. At that time also, I should use some addon to trace and broadcast warlocks' DoTs.

I minimize the 4 toons and use the main window in 1480 res.

I am using XP btw.