Vista Business 32bit.
Are you really running Vista32 with 8GB RAM? If you are then, quite honestly, whoever sold it to you should be shot.

How much video RAM do your two cards have (this will reduce the total memory available to a 32bit version of Windows)?

Have you looked in the performance monitor to see if there are any clues in there (what exactly is constraining you)?

How much physical memory does the task manager / performance monitor report to be available when you're running 3 WoWs? How much in total, too?

I wouldn't be at all surprised to find that you don't have enough accessible memory.