Here's a quick copy/paste from an interview w/ nvidia guys on nforcershq ..
So would a BIOS update enable 400 FSB support for those older boards?
NVIDIA has a BIOS that cleans up a lot of the "noise" and would allow some older boards to hit 400FSB. However, it is still up to the board maker if they are going to use that BIOS and validate their board for official 400FSB support. NVIDIA feels that the board makers will vary on their response to this. Aggressive companies such as ABIT will release the BIOS and state you can overclock on most of them and support it on others. More conservative companies - like Asus - will probably use the BIOS for stability, but will only certify the newest boards to support 400FSB officially or unofficially.