Do you have any games that would benefit from PhysX?
I would sell one of the cards, test out the one you keep with PhysX, wait for more reviews to come in on different cards used for PhysX (i.e. 8800GT vs. 8400), and then sell the GT if you can get the same performance from a super cheap card.
The only reason I can think of to sell both is if you can sell them as an SLi package to one buyer and make about the same amount of money that you would selling them seperately. Also saving you some shipping and the annoyance of two buyers.
-z