It's not a question of whether the GTX680 is capable or not, it's whether it's worth the extra money over the GTX670 or even the GTX660Ti, and the answer is no. The GTX670 has a price tag roughly £100 less and the performance drop is marginal. I can't really say about the GTX660Ti but I have heard it's marginally less powerful than the 670 for £50-60 less.
the i7-2600 with 8GB ram is plenty.