with a laptop display , how many more frame rates can we gain with an 280 compare to a 260 ?
It depends more on the resolution, not the screen size. If you are running a game at a low resolution (say 1280x1024) then the two cards are going to be pretty close in performance since that res is more CPU dependent. But as you start to increase the resolution and detail, the GTX 280 which has more shader processors is going to start to pull away. Based on notebookcheck.net's benchmarks, I'd say the GTX 280 can pull an additional 10-15 FPS over the GTX 260 on high detail. In a game like Crysis, that could mean the difference between smooth gameplay and choppy/not playable.