No need for a GTX 680 period, it costs 25% more on average than the GTX 670, and performs only about 5-6% better. Not a good cost to performance ratio at all, especially considering you can spend between them and get a 7970 GHZ edition that outperforms BOTH of them. .. But yea, the i7 does nothing for gaming performance that the i5 can't do.
I wouldn't do a 660 TI setup right out of the gate either, a single, stronger video card is almost always the best solution.
GPU: Two 660ti's are roughly 10-15% faster than a single 7970/680 but at the cost of a 20% cost hike and higher power consumption. I would rather get a single GTX680 or HD 7970 or save buck and split the middle ground with a GTX 670