Check this article on multimonitor support in Windows Vista
You're using a Heterogeneous Multi-adapter:
The term "heterogeneous multi-adapter" is used to refer to multiple graphics adapters using multiple graphics drivers in a single PC. A common example is the use of graphics adapters from two different manufacturers, each of which requires a different graphics driver from the respective manufacturer.
However, the Windows Vista Display Driver Model (WDDM) brings fundamental changes to the management of multiple graphics adapters and external displays. This includes a new restriction, because WDDM drivers do not support "heterogeneous multi-adapter" multi-monitor implementations. Specifically:
- All graphics adapters in a system must use the same display driver model. That is, all of them should either be running XPDM or WDDM. The driver models are mutually exclusive, and Windows Vista does not allow the simultaneous loading of both an XPDM driver and a WDDM driver.
If a system has one graphics adapter with a XPDM driver and another with a WDDM driver, then Windows Vista will choose the POST device, which is the one with VGA resources. This is commonly referred to as the "VGA adapter."
- If multiple graphics adapters are present in a system, all of them must use the same WDDM driver. If there are two graphics adapters with WDDM drivers from two different manufacturers, then Windows will disable one of them. The VGA adapter will be enabled, and the second device will be disabled.
Conclusion: get the Matrox, since it fixes your problem