I've got http://www.asrock.com/product/939Dual-SATA2.htm
which allows me to use both agp and pci-e graphics cards.
I have the same motherboard, and from what I understand, it should work(though I havent tried it myself).
I dont have that mobo but from what ive read it works by spoofing the agp over some of the pcie links. I dont think any chipset handles native agp and pcie, so they had to. I think you only have the equivalent of an agp x4 /pcie 4x in that slot. So theoretically there should probably be a full 16xpcie slot and it should work if the bios will allow it. unless the pcie switches over to the PEG