Multiple LCDs

I have 3 19" monitors I can only span 2 through my NVIDEA GeForce 6200 256MB have the 3rd running through a Radeon 7000 64MB PCI Graphics card. It works good to drag a window to any of the 3 monitors (1 standalone and 2 span together) but when running a full screen application in a window like Flight Sim as soon as I drag even a part of this into my standalone the whole program slows down to the point it almost freezes up completely. Is there a way to span 3 wide? Or will a 256 PCI boost the 3rd monitor for high graphics games?
Chris G