Well I hunted around and found the New PhysX Driver 8.07.18 You can download it here.
Download Nvidia PhysX Driver 8.07.18 Driver for Windows XP/Vista - Softpedia

I played around with all of the possible configurations and created a Graph for your viewing pleasure
In order to test the driver (in Vista) with just one card doing the PhysX calculations I had to use this work around. It should be fixed in a future Driver

Quote: Originally Posted by Guru3D.com 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 Vista users.