I'm an avid SC2 player and have been upgrading my computer ever since it came out.
I burned out my 8800GTX Ultra while running SC2 beta... I was able to RMA it for a GTX 280 (thank the lord), and I have since upgraded my CPU to an i7.
Now that the GTX 670 is out I was seriously thinking of dropping the money and getting one. I was actually at Fry's last night with an GTX 670 in my hands, but I opt'd to not get it. This is a money thing for me more than anything...
When it comes to GTX 670 vs GTX 680. I say the main difference between the two is not performance. It is quality of the hardware. While I'm not saying the 670 was made poorly nor am I saying that it may break, it is just that the 680 is Nvidia's flagship unit. The reference design of the hardware makes it cooler, quieter, and draw a bit less power in some benchmarks. In my mind it is similar to comparing a Corvette with a Camaro. They both may use the same engine, and get to 60 mph at about the same time, but the price difference buys you more than just power.
With that said, at ~$400-$500 no one is really buying either of these cards with a clear budget in mind. That is the case for me. If I was to be able to afford the $400 GTX670, I may not mind the extra $100 for the 680 if it is build that much better. People may think I'm crazy, but I am not. My GTX 280 is loud, loud!, LOUD! so much that it is a constant whine. While its very quiet, it is the most annoying sound that my computer makes. If I can pay $100 to get rid of this sound I'd do it today. For even my 6 year old GTX 280 is able to run D3, SC2 at really High FPS on extreme settings. It's not the top of the heap, but it is not sweating it either.