The three motherboards that will overclock right out of the box are the Asus A7V, the Abit KT7, and the MSI K7T Pro2.
The Asus board has dipswitches to allow for multiplier adjustment, the only thing is, not every A7V has these dipswitches. You have to make sure that the vendor that you get this board from mentions that it has the dipswitch multipliers.
The Abit KT7 uses Softmenu III and allows for multiplier adjustment in the bios. Also, if you get the KT7-RAID, you will get built in IDE RAID.
The MSI K7T Pro2 is a new motherboard that is, for the most part, based on the original MSI K7T Pro. The only major difference in the K7T Pro2 is that there are multiplier adjustments in the bios.
You can find reviews for all three of these boards here at anandtech.
As for unlocking the actual Duron you may get lucky and get one that has all of the L1 bridges connected, therefore, you will be able to stick the duron in the mobo put a nice HS/FAN on it and overclock till your heart's content! If the L1 "golden bridges" are not all connected, then you have to take a pencil, preferably mechanical with 0.5mm lead, though 0.7mm will work too, heck even a normal pencil will work really, and connect each golden bridge. The golden bridges are like this : : : : : and you want to connect each one individually so they look like this | | | | |. Then you can overclock. 🙂 Sometimes connecting the bridges with a pencil won't work the first time, if it doesn't, just erase the marks and try again. Or just go over them a little bit.
You can find more information about overclocking socket A chips in a review here at anandtech too. 🙂
Good luck!