The AGP bus' purpose in life is to transmit data from the CPU and RAM to the video card. The only time it is really used though is when textures are loaded off the HD (when a level loads for example), or when there are so many textures they don't fit into VRAM (and data must be swapped from RAM and VRAM).
Since textures are loaded off the HD between levels, overclocking the bus has no impact on gameplay. You might see a very small difference in level loading times, but nothing really huge.
Also, with modern video cards having 128MB of VRAM, they can hold a ton of textures on the card ready for use. So many that many games don't even fill it up! Thus, all the textures once loaded off the HD are resident in VRAM, and swapping will never occur.
Even if a game does have a ton of textures, they are usually loaded inteligently. This means old textures from locations you were at a while ago are unloaded to make room for textures you'll probably be encoutering in a few minutes. This makes it so that the swapping effect is minimized since the card will never need data in RAM immediatly. If it needs a texture, it will have been loaded a few minutes before with no real impact on gameplay.
the other day I was playing with the video settings on max payne 2, and set everything to max, and it told me I would run out of video memory (which would cause the game to crash?). I have a ati 9800np (128), and the message said I needed something like 146mb video ram. If the agp bus was overclocked is there more chance it could swap textures around fast enough to stop me running out of memory? Thanks