It's just sad to me that AGP was created so that PCI could be limited. I would think sending signals serially over two lines instead of one would be twice as nice, and so on. The fact that AGP has gotten such a wide-spread following, even just in the PC world, shows when it's done right, a single device slot is quite useful. AGP has right now is in the area of DVD playback. If you check the various software DVD players, many have difficulty playing at a decent frame rate over the pci bus as the data stream just about maxes the pci bus out.
Serial is much simpler, and therefore much faster in most cases.
Serial is not inherently faster. However, you have spent eight times as much on line drivers and receivers, and on the copper to interconnect them. Might be nice to load it in something less than the life of the universe. Go, AGP, go. Since the old days of the standard serial port, technology has increased enough to transfer data very fast on a few wires.