you can look cards like GT650. to be honest i think the second slot is better use for SLI. with 2 670 you will br able to handle more demanding games that will come inxthe future. also with such raw performance you don't need dedicated PhysX to play game with gpu accelerated PhysX in the future. but since you don't want to fork put anothet 300 for 670 then just look cheaper card fir physx. no nedf powerful card such as 580 for that purpose alone.