Playing today's average game, you will see very little improvement going from 4x to 8x AGP. Most folks don't seem to understand under what circumstances you can expect higher AGP speeds to help you.
People get confused when they see that high resolutions, with lots of AF and FSAA, do not get a performance increase with high AGP speed. That is because those functions are all about fill rate, frame buffer, and local memory bandwidth to access that frame buffer. You video cards brute force ability to render is tested.
high polygon counts and massive textures will benefit from AGP. If you have a very high polygon count, you will send alot of geometry information accross the AGP bus to the T&L engine on the Vid Card. There are games in existance that actually come pretty close to saturating the AGP bus in this fashion, and there are definitely professional applications which do so.
If your textures overflow the local texture memory on your card, you will see a big increase in speed going from 2x to 4x to 8x AGP. This doesn't happen very often. However, there is at least one, and perhaps more than one, game that I play that easily uses a few hundred megs of textures. Memory bandwidth, and AGP bandwidth begin to get inportant when you are constantly swapping 300megs of textures in and out of a 64 meg video card.
I hope that some folks will read this and understand that they are not going to get high 3Dmarks because of AGP 8x, but when you are rendering in the millions of polygons per second, and you are using 512MB of textures, you are going to notice that AGP8x.
It is actually under these sort of conditions that SBA and Fastwrites show their value. Under most conditions SBA and Fastwrites are not worth the headaches, but under sever stress of the AGP bus, they are.
Hope this will help folks with their "AGP decisions".