That doesn't exist. AGP slots use less physical lanes than a PCI-e slot anyways, so even if it did, the PCI-e slot would perform like an AGP card. Sorry. You can get PCI-e motherboards for fairly cheap nowadays, though...
Newer AGP highend cards are basically PCIe cards with a 'bridge adapter' to allow it to run on an AGP port. THis adapter though, cannot be purchased separately from the OEM combination with video card.
they exist, in a sense that they were made (at least prototypes). I've never seen one for sale though, and only 1 obscure article written in Chinese talked about these adapters, and had a pic of them. That was back in 2005 that I found that info. It's long gone by now, but there was such a contraption devised. You could only use half height cards with the adapter of course, making it pretty worthless.
they exist, in a sense that they were made (at least prototypes).
Albatros announced they would produce a "riser" adapter a couple of years ago, but it never went beyond the prototype stage.
An alternative is to buy a motherboard with both AGP and PCI-e slots. Asrock (subdivision of Asus) makes such mobos, but they generally use older generation chipsets so you will not be able to install any of the 45nm Core 2 Duo CPUs made by Intel. They will also not support any type of Quad Core CPU regardless if it is an older or new generation of the Core 2 Duo CPU.
More importantly the PCI-e slot is limited to x4 lanes instead of x16 lanes. While x4 lanes will suffice for entry level/mainstream card, having so few lanes will definitely bottleneck a 8800 GT or GTS (I think I've read a 25% performance loss).
My advice, ditch AGP and simply get a PCI-e video card.
jaguar, the Asrock mobo should support quadcore Kentsfields, but everything else you say is right on the money. Too old of a board for future generation CPUs, plus it's VIA, with all their inherent downsides.