While going to slower memory timings may allow a higher fsb, overall performance will likely suffer - the memory timing penalty for 47.5 mhz isn't worth it, imho... There's some point where it balances out, but that's way above 138fsb......
Try it anyway, the worst thing that wiil happen is that you'll scramble the registry, have to reload, and that's unlikely.... you won't break anything..... push it up 'til it doesn't work, then clear the cmos, back off, be happy....