Thanks for your answers!

I tried to Run WoW 5 times on this system:
Main Display -> 1x 1600x1200 (windowed) with lowest detail/resolution settings
Left/Right Display > each 2x 800x600 (windowed) with lowest detail/resolution settings

Screenshot:


Result: If i minimize the Clients on the left/right Display it works (with ~30/40 fps on the main). If i open all 5 instances, the framerate is below 10 fps (unplayable). If i enter a City it`s even more horrible!

Fact: With my current setup 5 boxing is impossible
Any ideas whats wrong (told me it should work!)

Additional information:
Windows Vista x32 used (need to upgrade to x64 to use all advantages and even the whole RAM, currently only 3 GB are supported).
While i run (5 times) WoW, the Windows Task-Manager shows me a 100% CPU- and ~40% RAM usage).

Thank you!