Well, on some games, the phenom does pretty well so upgrading to a GTX670 is a good route. On the other hand, some games run quite poorly on AMD CPUs that upgrading to a new platform might be the sensible choice.
For BF3 though, it almost doesn't matter what CPU you use (I'm talking about single player) since it's a very well coded game and the frostbite 2 engine can utilize up to 8 cores (if I'm not wrong) so I recommend upgrading your video card. You can always OC your CPU later if you feel like it's not performing well enough in some other games (ie. Assassin's Creed 3, which runs quite poorly on AMD CPUs)