I had an old Asus sp97-v mobo that I had an AMD K6-2 500MHz CPU in. I used the 2X setting, which remaps to 6X, and the mobo has an 83MHz FSB jumper setting(6X83MHz=498). The Asus mobo also has a Async jumper to keep the PCI bus from being overclocked. If your mobo has the same options you should be able to hit 500MHz. I know the K6-2 500MHz prices have came down alot, so even if it doesn't work you can underclock it to 400MHz.
Good luck,
Stang289