I have a Radeon 5750 under Win 7 Pro. I have read that in order to run 3 displays (this is an office system, not gaming), that "Driver version 8.66 (Catalyst 9.10) or above is required to support ATI Eyefinity technology and to enable a third display you require one panel with a DisplayPort connector" are the requirements.
The only way I can get the 3rd display up is to disable one of the other 2. However I'm using a Display Port to DVI dongle. Does anyone know if this should work? Is the only way to go to a LCD with a Display Port?