Hang on... let me get this straight. You want an AM2 motherboard with support for DDR2 and AGP, because you have an AGP card lying around. Tell me again why you can't sell your AGP card, buy the PCI-E version, and instead get an AM2 motherboard with support for PCI-E and DDR2.
You're buying a new motherboard, not sure where the rest of the "I don't want a complete overhaul" part comes in.