Nouveau is a project to develop free drivers for the NVIDIA video cards. NVIDIA itself only provides proprietary 3D and obfuscated 2D drivers.
See Games Compatibility for information on how games currently work with NV40 cards and git builds. Note that there currently is no tech support for 3D nouveau users from the developers.
Texture compression is still unsupported by the new Mesa, and nouveau doesn't yet use a memory manager, so most problems are with games that use lots of textures.