Gotta second Big Lar, here. If you absolutely have to OC, 'fraid that an Intel manufactured mainboard must usually be replaced. Intel makes high quality mobo's, it's too bad that they don't OC worth beans. Perhaps the "Seattle 3" mainboard DOES OC, don't give up the ship! Flashing mobos with foreign Bios's is a high risk undertaking.
If you'll have to get a new mobo, a new foundation for a Duron/T-Bird system is worth serious consideration.