recently released a couple slides evaluating the differences in performance between the current 65 nanometer Geforce GTX 280 and its soon to be released 55 nanometer successor, the Geforce GTX 285.
As predicted, the average performance gains seem to be on par with previous assumptions, with the GTX 285 giving roughly a 10% performance boost over the GTX 280 in current mainstream titles such as Crysis Warhead, Far Cry 2 and Call of Duty World at War.
According to several sources, the GTX 285 will be clocked at a 648MHz core, 1476MHz shaders, and 2484MHz of GDDR3 memory. It can be inferred that these improvements in clock speed due to the 55nm fabrication process will allow for noticeable increases in overclocking ability over the GTX 280. As a result, these cards should benefit from more noteworthy performance once they hit the retail market on January 8th alongside Nvidia's dual PCB 55nm GTX 295.
You can find the mentioned slides here