Personally I think for your use, I would be more inclined to replace 1 of the existing cards for a more powerful one.
Replacing won't work if he replaces with a NVidia, he can only run 3 monitors off the GTX295 because its a dual GPU card. That's why I suggested adding a 3rd card and using it for the main monitor only. Replacing with an ATI will work but you have the aforementioned display port adaptor problem. Also if his connects his displays correctly he will be able to free up one of the GTX295 cores and dedicate it to PhysX if he wanted.
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