Most of the time its still (somewhere) on manufacturer's sites.
The motherboard I have *might* go to 400 - I'd suspect that they will, because its really a matter of what the clock multiplier is. Most (if you do the math) can go much higher than they say in the manual (with the settings already given to you). Breaking out the calulator and taking the DIP/jumper settings raw, many times you could go "higher".
Might also want to check out Yahoo! auctions.
Addison