NVIDIA's GeForce PhysX implementation
I have to be honest here, all respect to the PhysX team for making this happen. NVIDIA has created three sets of circumstances on how you can choose to use your PhysX setup from within the PhysX driver, let's have a look:
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Standard - one GPU renders both Graphics + PhysX (not ideal as you'll need a lot of GPU horsepower).
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SLI mode - have two GPUs render both Graphics + PhysX.
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Multi-GPU mode - GPU1 renders Graphics and GPU2 renders PhysX.
For me personally the last option is by far ideal as this is a situation where with any mainboard with two x8 or x16 PCIe slots you can use your old adapter as PhysX unit.

See, you do not need a power-house of a graphics card to deal with the PhysX calculations. Say you have an older GeForce 9600 GT or 8600 GT lying around and you upgraded towards a GeForce GTX 260 (or whatever). Then can you use your 9600 GT (or even 8600 GT) as a Physx unit and the GTX 260 for graphics. Why do I like that so much? Because you can do something new with an old outdated graphics card and ... and you are not bound to an nForce platform since you are not running SLI mode.
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Most IMPORTANT
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.
In other words you do not need to have 2 monitors in XP, while in Vista you need 2 connections or you can bypass that with above trick.
With a single card or two cards in SLI mode you will not have this problem.