Well, the question you ask is really a 2 fold question:
Which chipset is the best overclocker?
Which board gives the best settings for OCing?
The answer to the first is the nForce2 chipset. The KT333 and 400 are decent, but not as good.
The answer to the second is going to be either Abit, Asus, or MSI. All of these manufacturers give a lot of options in the BIOS for overclocking. Of the 3, the MSI will be the best value.
So, I'd look for MSI boards with nforce2 chipsets and get the options on the board you want (video, SATA/IDE, sound, NIC, USB etc).
Then when you find the board, go to
www.pricewatch.com to find the best price and cross-reference the dealer at
www.resellerratings.com .