I would say the 670 would be the better to the two but it is so close in benchs that you probably won't see much increase in preformance. T
he only way I can show you in comparison is to show the 5850 CF vs the 580 then the 580 vs 670 and you will see how close it is.
So I would probly keep the 5850's for now and get the best usage out of them and when the games start slowing down then invest in a new card or cards. Good luck to you.
5850 CF vs 580http://www.anandtech.com/bench/Product/303?vs=305
580 vs 670http://www.anandtech.com/bench/Product/517?vs=598