According to the Inquirer, the best selling add-in video card last year was a PCI FX5200, and PCI cards formed a large part of the top ten selling cards (I forget the exact stats, but you can look them up on the Inq if you're interested). That's probably just for retail sales of boxed cards rather than total sales to system builders etc, but there clearly is a big market for PCI cards still. However, the vast majority of people wanting an add-in PCI card are going to want a £50-80 one rather than something of 6600GT/X700Pro level, which is why I guess there are no PCI cards featuring higher end GPUs.
There's no technical reason why it wouldn't work, as far as I can tell those boards based on 915 chipsets which have a sort of 'quasi-AGP' slot as well are just running an AGP card off the PCI bus. You lose 10-20% performance over running at AGP8x as I remember.