Depends on what games you play. If you play Physx supported games (Arma III, Batman, Borderlands 2, Metro etc) then you should keep the GTX 260. If not, then just sell both 260 and put it towards another GTX 760.
There aren't any driving games that make use of GPU accelerated PhysX. Make sure you have some to play before installing the dedicated PhysX card. Here's a list of games: http://physxinfo.com/index.php?p=gam