Most of the gains you would get from switching to the GTX 670 would be gaming-based (i.e. better graphics and such). Here is a comparison between the two: http://www.anandtech.com/bench/Product/548?vs=598
If you look down though, compute-based activities, the 7870 is better than the 670. Radeon-based cards are typically better in this area. For this reason, I would suggest that you stick with Radeon cards.