benchmarks only tell maximum FPS they never tell the whole story.
you should have listed all your hardware, your monitor and res and what games you like to play or will be playing.
I would say the 670 is an upgrade, but not a very viable one. It would only yield minimal increases in performance, and at a 400+ price point, maybe not.
I have a amd 6 core processor, 2 750 seagate harddrives, corsair 850 watt psu, and the 2 6850s. 21.5 inch 1920x1080 res. I like playing l4d2 and what I have now is sufficient. I'll want to be able to play borderlands 2 and dead space 3 on full settings. It'd be nice to play all my games at full settings.