Im currently running all 5 of my toons on 1 Mac Pro, although its the new dual Quad-Core.

I would guess that your dual Dual-Core Mac Pro would run 5 ok with enough RAM. How much do you currently have in there? I had 4 when I started, lagged quite a bit in Capital Cities... upgraded to 8 and no issues anymore... a little lag on initial load in cities, but manageable.

You should go ahead and sign up for the free trial accounts and give it a shot.

Also, I am running the 4 additional accounts as Symbolic Links off of the main account.