This is sort of a pre-[EDIT]. I thought the 670 was a beefier card than it is, but the 660ti and 670 look identical in specs (same CUDA, same stock speed). The 670 has a slightly larger memory bus (192bit vs 256bit). Given that, and assuming mem bandwidth doesn't help those things (which I dont think it does) then the 660ti might be a better choice then.
This system is already well above the specs needed for good gaming. It's sort of a non-issue for gaming at this point. The tweaking is really getting best bang for buck for rendering, by now.And yes my main goal is rendering/designing but I will also want to play as I tend to PvP quite a lot.