I suppose you have the regular Asus P2B.
It depends on your revision. If you have the 1.02 revision you can only use the Katmai-core to 600 Mhz. If you have a newer revision you can use the cuppermine CPU's.
You can use the coppermine CPU's even on the older MB's by using the product found on www.powerleap.com.
You could also take a chance and plug in the coppermine anyway and set your voltage to the lowest possible: 1,8V.
This voltage is too high off course, but bare in mind that is the same voltage the illusive PIII-1.13 Ghz should have.
With decent cooling it should be possible (haven't tried it myself, so take this info for what it's worth, I personally bought an AMD 1Ghz).
Hope this helps.