As far as I understand it Tom there is no requirement to have a PhysX Physics Accelerator Card installed on your PC in order to use the PhysX SDK and most of it's physics functionality. It's just that without one all of the calculations are being done on your computer's CPU rather than being run on the PhysX card. As a result the overall performance is likely to be slower.
The PhysX SDK is freely available once you have registered with Agiea and can be used royalty free for commercial or non commercial games for the PC. The licence is available for viewing on their site. This appears to be a strategy on their part to encourage developers to use their Physics engine with the hope that this will boost sales of their hardware.
TGC's Dark Physics pluggin is I believe a higher level encapsulation of some of the PhysX functionality presumably making it easier for most people to integrate Physics into their projects.
I have the SDK myself but have been using ODE in my Dark GDK projects so far and have not had time to explore PhysX. It does however appear to be a lot more powerful and I hope to evaluate it in the not to distant future
No matter how good your code is, someone will improve on it
