The only micro-atx board I've used with a Duron that has good overclocking abilities is the Soltek 75-KIV I believe. I bought mine from soltekusa.safeshopper.com, the authorized US distributor for Soltek motherboards. It has jumpers for CPU clock multiplier and is based on the KT133A chipset. You can tweak the FSB in the BIOS and it was one of the early boards that has the "red storm overclocking" feature. You might as well just overclock the standard way as the red storm feature will put you at somewhere well below the stable limit (in my experience with this board). It works well though I haven't pushed the board much (it's running linux and just sits in an office most of the day

).
I checked the soltek usa site and they don't seem to have the board at the moment but as I recall they were fairly quick to respond via email. I know some vendors that list on pricewatch carry Soltek boards too.
Asus has a micro-atx kt133a board too I think...
Also if you can wait a little bit, Abit has their NV7M micro-atx nForce motherboard due out sometime this month. It looks like it'll make a great, high performing micro-atx board for an AMD CPU.
Let us know what you choose!
Gaidin