Don't know if this helps:
A friend tried to install a PCI Radeon (don't know exactly which one, must have been one of 7000/7200/7500) on an Asus P55T2P4 (Intel HX).
The system didn't post.
The card worked fine in my system (Gigabyte GA-5SG100, SiS chipset, had an AGP slot).

I don't know what the problem was, might even have been the BIOS.

I suppose you could indeed get higher fps with later cards because hardware T&L could help a bit for OpenGL games. A Radeon 7000 does not have T&L yet, though.

Could be a hardware incompatibility. This Rage 128 VR I have does not even turn on in some PIII machines, but works fine in my 486.

The best chance of getting Radeon drivers work on a 486 is probably to start with one of the first Radeons and work my way up if the base model works. With which PCI Radeon did hardware T&L first come about?

The Radeon VE aka Radeon 7000 (RV100) is the only model that does not have T&L.
It came later than the original Radeon SDR/DDR (aka Radeon 7200) so you might try that one first.