Your supposition is basically correct, jolt.
The standard AGP connector is 32bit data width as indicated. There was a 64bit slot used on very high-spec professional video cards. But the bus width is only part of the story, the bandwidth (Data Transfer Rate) is enhanced, in comparison with the PCI slot AGP is based on, by running at 66MHz, and using multiple clocking.
Whether the data bus width between the video chipset and the on-card video memory is 64 or 128 bit is not directly related to the AGP bus connector bitwidth. A 64 or 128 bit memory bus will contribute to the DTR between the video chipset and that on-card memory, and this can be expected to be much higher than the AGP port DTR.
When the card needs to access system memory or obtain instructions from the CPU, this is done through the AGP port, so yes, at those times the bandwidth is restricted by the 32bit AGP slot. This means there would not be as much performance improvement between the two (64bit & 128bit video memory bus) as could be expected on an 8x AGP system, as the 4x AGP port will be something of a bottleneck.
Edited by Platypus, 11 August 2008 - 10:18 PM.