it really depends on the game you are playing some games are very light and will work at 3D ok, and other you may nee to lower some settings, also be aware that not all games works properly in 3D.
for 1080p gaming you will be allright dude. i am using a gtx 680 and the performance in awesome i also play in 3D on a 1080p Benq XL2410T if the 670 is really so close to the 680 you will be allright, just make sure you downloaded the latest drivers 304.48 they increased a lot my performance on 3D Gaming.
a single 670 can definately do 3D. but if you want to be doing it at high or ultra settings, you will probably be pulling 2X AA max. though it can be argued that in 3D, aliasing is less of a problem.
the reason is that 3D effectively halves your frame-rate. and for most people, I like to average at least 50fps, definately not let it drop to low 30s. in which case the game would need to run at 80fps+ minimum in 2D for the game to be enjoyable in 3D.