Yes and no.
In theory it does backward compatible, but the fact is some PCI-E 1.x motherboards need to update BIOS to recognize pcie2.1 AMD card. And if you have the old MB that don' have a bios update then it can't detect the card.
I check your MB bios version, it has the newer version came out 7/28/2008. Your MB has the G33 chipset that came out Q207. So it may or may not compatible with the pcie2.1 card. If you already have the card just install it to see you can boot the PC or not. If you don't buy the card yet, you are better to contact ASUS to ask the BIOS info or buy the pcie2.0 or3.0 card instead the pcie2.1 card.