Yep, all 3dfx AGP Cards would
greatly benefit from AGP Texturing (just remember all the GTA3 problems due to the 16MB limit of a Voodoo3).
...and such Problems will just get worse, as the Banshee's and Voodoo3's 16MB are getting a severe Bottleneck more and more.
Anytime a Voodoo3/4/5 has to swap Textures via the classic, slow PCI variant (some Drivers contain this feature, but not all), the framerates take a massive dump (sheer unplayable), whereas 2x/4x AGP would allow direct access to the AGP Texure Memory (or permit direct Texture Manipulation) at higher speeds.
Even modern Cards can exceed their local Video Memory by a fair margin (e.g. high FSAA used in conjunction with large Texture amounts), and AGP Texturing just helps making the swapping at least as fast as possible.
That's still significantly slower than local Video RAM, but at least alot faster then via PCI66.
I'd say, 4x AGP in hard situations (e.g. 32MB total used, but only 16MB local Video RAM available) would make the difference between 0.5fps and ~10fps (depending on amount of Multitexturing, Bumpmapping or other Texture Manipulation involved).
Not much, but better than nothing.
In the past, 16 or 32MB local Video RAM was more than enough (and at the same time, 1x or even 2x/4x AGP was a practical joke due to insufficient System RAM bandwidth), but these times are about over