well, first off... _can_ you boost the AGP on your motherboard?
Secondly, if you're running 4x or 8x AGP, you're probably not really taxing the AGP bus and have plenty of bandwidth, so boosting AGP frequency wouldn't offer up too much of a bennefit.
Of course, if you have a mobo that can do it, and you want that last percent or two, go for it & see if your gfx card will take it. When I had my GF2mx, I could run the AGP with at 150% of stock without any ill effects; GF3 choked on it.