Firstly GeForce 7800 is NV47, not NV50. NV50 series is GF8 and up. All cards that get dropped from driver support now are in the NV4x range.
Second: we (Nouveau) got a new gallium driver for the NV3x and NV4x range of cards, which is a great improvement over the old driver. It passes far more of the Piglit OpenGL conformance tests and provides far better performance. You can get it in Mesa git or Mesa 8.1/9.0 whatever it will be called, once it gets released.