I doubt you will see any significant performance difference between AGP8x and AGP4x. Even 2x won't make that much difference.
And GLXGEARS IS NOT A FREAKIN' BENCHMARK
Seriously, all glxgears does is draw flat-shaded triangles and clear the screen a lot. No textures, no shaders, none of the things that modern GPUs spend their transistor budget on. An old card will usually outrun a new card on that kind of test.