If you're not looking for smokin fast, I know the Duron 1.1g CPUs don't have the L5 bridge cut, so they do SMP out of the box.
I recently put together a dual 1.1 duron box pretty cheap. I was impressed too, it was quite peppy.
www.knowledgemicro.com has a good deal on the durons, and they also had a pretty decent MP price if I remember correctly.