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rigid body these days can be fairly decently threaded on any decent CPU.To the best of my knowledge the hacked drivers has not been continued for a very long time. I tried those back when 7950 were still popular with a 560 ti and the performance was horrible. Even today physx doesnt seem to be that great to offload unto another gpu unless that gpu isnt fare from your main nvidia renderer in terms of raw performance. Nowadays everything physx related is version 3 (i think) which does run somewhat alright on the cpu.
Is funny how the stubbornness of nvidia has manage to pretty much reck their own tech in terms of use and wide adoption.
Most software PhysX can be offloaded to the gpu with some tinkering but i have yet to see any performance improvement from doing this. Might be alright if your cpu is ancient but im sure other things will bottleneck first in that case.