Dishdog, and Fisher, I your getting confused with the 8x bus on AGP which is the maximum yes...
Though Pixel Pipelines are completly different and are to do with the card.
They are what power the 3D graphics, as each part (textures, etc) are split into these pipelines which are then rejoined as they exit into one image.
The more you have, the better the quality, resolutions you'll be able to run at.
So, I don't see why AGP cards can't have 16 pixel pipelines if its handled from within the card. That link is a perfect example