I'll have to look. Most of the available Opteron motherboards currently available do not have AGP, and are aimed at the server market rather than workstation. I gotta look them up for some exact links, but RioWorks (
http://www.rioworks.com/HDAMB.htm) does have a dual Opteron board with AGP. MSI has a dual opteron board based on the Via 8KT800 chipset, but it only has 4 DDR slots, though it does have AGP. Asus SK8N nForce 3 Pro 150 is the only single Opteron board available, but this again has only 4 DDR slots.
Right now the Opteron is a primarily a server-class CPU, and most of what you're gonna find in the way of server boards will have at least 6 DDR slots, but no AGP. Right now, the RioWorks board is the only one that fits your criteria. I'll have to look around, but it doesn't seem to be available in the US, according to Pricewatch
It's new technology and isn't targeted at the desktop. You're also looking at around $300-500 for a basic motherboard, and like $250 a pop for each CPU.