1) the 3500's 16-bit sucks compared to the 5500's 16-bit
2) Glide doesn't "stink", it just hasn't been updated in years. It could easily be updated to support 32-bit color, larger textures, etc. just like any other API, but there is no reason to do that anymore.
3) Glide was specifically written to work with 3dfx's hardware architecture. It wouldn't just "work" on nvidia cards with simple driver updates. The glide wrappers prove this. They suck.
As far as glide being "dead", it is still the most stable and consistently fastest way to play UT, Rune and Deux Ex, 3 of the more popular games out there. The D3d patch for Deux Ex makes it playable on a GTS, but a 3500 will still wax a GTS-U in Deux Ex. Rune doesn't seem to be quite as "glide-dependant", but it still plays much better under glide than D3d.
OGL in UT is still far slower than glide, even with the new patch, and it's not as stable. Small anomalies pop up.
that being said, with the D3d DXTC patch, D3d 32-bit looks pretty damn good with the 5500 using the ocmpressed textures. Not quite as fast or as smooth, but still pretty darn cool. Some weirdness (flags), but pretty good nonetheless.
Glide still rules for racing and flight sims, period. =)