We learned that at least a few Nvidia partners will first go for non overclocked cards. No one wants the Osborne effects where you have a cool product and announce even better one and cripple the sales of your original part.
The first G84 cards branded as Geforce 8600 GTS will work at 675 MHz core / 2000 MHz memory. The later versions will work at more than 700 MHz for the core and even more aggressive memory speed.
None overclocked comes first and plans to take the market and after than when ATI finally launches RV630 based Radeon X2600 series Nvidia and its partners will answer with even faster clock.
There should be more place for overclocking these chips as they are 80 nanometre. The Nvidia reference Geforce 8600GTS is suppose to launch on the April 17th.