I tried it for a time last year with a 3 x 7970 + GT 650. It turned out (for me) to not be very useful and more trouble than it was worth. If you look up the list of games that actually make use of PhysX, how many of them do you actually play. that might be a good indicator of weather it is worth it for you.
Another issue is that they are constantly having to work around (hack) to keep up, and if you want certain benchmarks to work, you have to use a very old driver set.
You can try it here
http://www.ngohq.com/graphic-cards/17706-hybrid-physx-mod-v1-03-v1-05ff.html
*** opinion coming up*** it really is more maintenance than it's worth and if you read the comment in the support forum below, you can se that there is an awful lot that that can go wrong, and exceptions that do not work with this kind of setup.
here is a list of PhysX enabled games
http://en.wikipedia.org/wiki/List_of_games_with_hardware-accelerated_PhysX_support
Not trying to be a downer, but if you have not yet purchased a card yet, you may want to do some reading and look at other folks experiences before investing.
There is an alternative called 'Hybridz', but this comes with a list of exceptions, and non-compatibilities as well.
http://www.ngohq.com/graphic-cards/...with-latest-physx-and-geforce-285-solved.html
If you do decide to go with it, on a AMD hybrid system A GT640/650 (or the equivalent) is enough to handle the PhysX. You do not need a 770 lets say. It is at least inexpensive if not very flexible and inclusive .
Good luck