chris_shadez :
280 beats the 670 and is a lot cheaper too
try this link to compare GPU cards, just adjust the card model numbers in the address bar to compare results quicker rather than actually searching for the specific cards to pair up
http://gpuboss.com/gpus/Radeon-R9-280-vs-GeForce-GTX-670
The question is: If you already own the 670 is it worth upgrading to a 280? That answer is no. Earlier I suggested making an upgrade to a 290. That would be a big jump in performance and a lot more VRAM.
Also, I'd like to suggest when making comparisons, don't use gpuboss. It only lists specs and isn't really useful in terms of real world performance. To really compare GPUs you have to look at reviews and benchmarks that compare different GPUs in different scenarios such as Battlefield 4, Far Cry 3, and synthetics.