I think that the 790 series fares better in general with overclocking, but having not owned an nvidia board since the 570SLI, i cannot comment personally. These are generally more expensive that the 780, but i think if you want the best chance of overclocking then its probably your best bet. There seems to be mixed views on the 780 series - some people seem to have a lot of luck and others none, particularly with the asus striker series. There have been lots of failures with these boards, but again i think there have been people who have had a great deal of success.
The 680 is an old chipset, and its probably not worth considering unless you can find one for a low price. Rainmaker is probably right in that if you want the best chance of overclocking a quad, your better off with an intel chipset. However, i have heard that the EVGA *** series of boards are pretty good - the 750i *** gets a good review on xtreme systems, and seems to be fairly capable with quads. I think now there is a 780 ***, so that could be worth looking at. A lot of it comes down to budget - i think most people would agree that the 790 is the best overclocker, but it is pricier.
