lol 80% increase! That would be sweet, but obviously that is a pipe dream!
It will be a new architecture, which generally means a 20-30% increase in performance for the same TDP over the previous one. The performance gains from the 480 to 580 was minimal because it is the same architecture. It was the same story from the 8800, 9800, and 2x0, as they were all very similar cards (and the 8800 and 9800 were identical). The 680 will be a new architecture, and we are expecting a sizeable performance gain per TDP over the 580.
Personally I like looking at the bleeding edge and backing off a little. The best longevity/$ is somewhere in the 560TI to 570 range, but the new AMD cards will be out this month, and the new nVidia cards will be out (and will likely beat AMD) in April, so I would be hard pressed to buy a high end card now when a new high end is literally right around the corner.