AGP-Speed Benchmark Results - 3DMark2000
This test is sending a large stream of triangle-data to GeForce, thus taking advantage of AGP. Keeping the light sources as low as possible makes sure that GeForce's lighting-engine isn't becoming the bottleneck in this benchmark. You can see that there's a vast difference between AGP1x and AGP2x. The step from AGP2x to AGP4x however is rather small, probably due to a limitation of either the CPU, GeForce's transform-engine or fill rate.
AGP-Speed Benchmark Results - SphereMark
The results of SphereMark are very similar to the ones from above. Again this benchmark is mainly sending many triangles over the AGP to the GeForce-chip. It's easy to see that AGP1x definitely doesn't cut it.