Crossfire can have stuttering, but there are ways to fix it with ease.
Don't waste your money on a 680; it's only 5% more powerful than a 670, which isn't worth the $100 premium.
I have a 3570k and an EVGA 670, both overclocked, and I can max out most games out there at 120fps. The ones I can't are either horribly coded (Like metro 2033), or require selective turn-downs, such as supersampling on the Witcher 2 or shadows on BF3... nothing that makes a noticeable difference, graphically.
You're going to get amazing performance out of either a 670 or 7970.