NV1x: Geforce256, Geforce2
NV25+: Geforce4 Ti
NV30 are Geforce 5, indeed. They sport a NV2+ mode, which will be unused in Gallium in favour of the programmable shaders.
There was, at one time, talks about using an interpreter to use Gallium3D on NV2X. Current Mesa code uses NV2x' NV1x compatibility mode (which allows the use of NV1x driver code on NV2x, but of course, limited to NV1x capabilities). In essence, NV2x cards are underused.
That was some time ago though, I wonder what's up with that.