firstly, physx is now enabled on all nvidia cards from what I can remember. 8800 on up.
the 640 is too slow to run the graphics and the physx together. but running it as a physx card with a higher end nvidia card is the way to go.............
but, if you run a higher end card you might not need a separate physx card because the card will probably be able to handle it anyways.
depending on the cards........... if you run a physx dedicated card (m 640 )...... it may retard the performance of the system vs not having one at all.......... to a certain point ( i think the 540 card?? ) was about the lowest you could go before it made poorer performance.