> BTW I think I heard about some motherboard which had two AGP slots, but the second one was AGP only physically/electrically, running over a standard PCI bus. But maybe my brain is just making things...

I've not personally ever seen a board with dual AGP slots, but there were a number of AGP and PCI-e supporting oddballs during the transition period. I recall one of the more terrible ones doing something like basically just allowing an AGP card to hang off the PCI bus. There were some AGP/PCIe chipsets that were quite good during this time as well, but many of them seemed to be crappy hacks with performance limitations or compatibility problems.

Also interesting were the graphics cards that used Nvidia's AGP -> PCIe adapter chip which allowed them to keep selling older hardware on newer platforms.