I say do it. Where do you come up with $500 btw? 670's aren't that much.
Anyway, a single 670 isn't enough to keep games like Bf3 out of the 30's. You have likely noticed frequent dips into the 40's quite a lot. Two 670's is much better, and while you will be CPU limited in some of those games, having the GPU headroom is great and will ensure you get the most your CPU can put out. This headroom will also come in handy for upcoming games, like BF4, crysis 3, far cry 3 etc.
When i'm playing BF3, the GPUs are limited by CPU, but as soon as a ton of explosions hit the screen, the GPU's jump to 95% each and framerate holds rock steady. That head room is being used for those situations. It is these situation that a single card tanks to lower 40's / upper 30's.