The potential of AGPx8 is vast as it operates on a 32 bit channel octuple pumped(66 MHz) churning a total of 533 MHz which can potentially deliver a data rate of 2133 MB/s.
AGPx4 operates on a quadruple pumped channel with a total of 266 MHz and can deliver a rate of 1066 MB/s.

But as is already mentioned in this thread, games seldom need more than 2x AGP which churns out data at a rate of 533 MB/s… that is megabyte!!
There is seldom anything gained by increasing your bus deliverance on your graphics card. This is not the typical bottleneck.