The next line of the Nvidia GPUs would be the GTX 270/290 which are essentially the same as the GTX 260/280, but with a die shrink. I think it would be out around November 2009 since the 260/280s were only released over the summer. Waiting would still help since the GTX 280 already fell by like $250 since its launch

Currently, the best powerhouse out there is the Radeon 4870 X2 which is available for desktops and should be available on laptops in a few months.