The T-Buffer and 3dfx's RGSS FSAA doesn't follow API specifications, it bypasses it. Same goes with 3dfx's 22bit rendering. It has nothing to do with the API, it's done by the T-Buffer unit. I'm guessing 3dfx and the use of the M-Buffer will add some really nifty effects that can be done in hardware without having to be coded. I guess we'd all look pretty dumb.
Rampage is completely new architecture.
The VSA-100 is really an upgraded version of the Voodoo3 core, which itself is just an upgraded version of the Voodoo2 core, which itself again is just a rehash of the Voodoo1 core. There will most likely be some key features found in Rampage that won't be able to be performed by the NV20 chip, such as QuadTexture rendering in a single pass. I think some of the features found in Rampage won't be available until DX9 comes out, but that's probably because they couldn't get it into DX8. Hehe, go figure. Either way, you can expect the big features found in todays hardware, other than a Voodoo5 of course, that will be found on Rampage.