In this article, Guru3D explains how to use Multi-GPU with NVIDIA PhysX under Windows XP and Vista. There are 3 ways to use GeForce GPU with PhysX:

• Standard – one GPU renders both graphics + PhysX (not ideal as you’ll need a lot of GPU horsepower).
• SLI mode – have two GPUs render both graphics + PhysX (SLI motherboard required)
• Multi-GPU mode – GPU1 renders graphics and GPU2 renders PhysX (SLI motherboard not required)

Now the trick to use Multi-GPU mode with Vista:

Now there’s a thing you will need to be aware of in the Multi GPU mode, it’s actually a Vista limitation but a second monitor must be attached to enable PhysX running on the second GeForce GPU. You must extend your Windows Vista desktop onto that monitor.

To bypass that issue, most monitors have a standard VGA and a DVI connector, right? Just use both. This limitation is related to the Windows Vista display driver model (WDDM). This limitation does not exist in Windows XP. In NVIDIA’s upcoming drivers, they will be offering a workaround to improve the experience for Windows Vista users.

With a single card or two cards in SLI mode you will not have this problem.

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