I highly doubt nVidia is going to release any new midrange parts anytime soon. If the HD 2600XT outperformed the 8600GTS, we'd see nVidia with a new card, likely. However, it is not faster and nVidia has no reason to launch a new card.
I think the next time you are going to see a new release from nVidia is with the GeForce 9 series, and we've still got a few months (at least) until that comes. There's simply no market (except for the extreme low-end, perhaps) that nVidia is not doing well in with its current lineup.