Several people have been using this form to find out about building their own laptops. With this comes the desire to overclock and under volt the Pentium m. This is a quick guide to doing just that.
For those who have a laptop based on the 915 chip set and have a 400mhz FSB processor there is a simple wire modification that will force your processor to run at 533. I do not recommend getting a processor with over a 17 multiplier as there is not guarantee that it will boot. So your best options are the 735 which should do 2.26ghz and the 725 which should do 2.13ghz.

The first thing your going to need to do is make the wire for this mod. I use a single strand from some braided speaker wire. Cut it to a length of about 4-5mm. Then bend it into a U shape from the middle of the wire. I find that the best way to insert the wire is using a small pair of needle nose pliers.

Now that you have the wire insert it into the two holes indicated in the picture. As you can see I have already placed my wire in the correct place to assist you in finding it on your laptop. Now pop your Pentium M into its socket and lets boot the sucker.

Now that you have it modded to run at 533FSB your laptop is going to use allot more battery power. That's where the undervolting comes in. Download .NET and Centreno Hardware Control (CHC). This program lets you set up voltages and multipliers. I use dynamic switching so that as the computer requires more power it simple uses one of my preset multipliers and voltages.
To test use prime95 set it to torture test large FFT and let it run at the default voltage for a while, then using CHC reduce the voltage, again let prime95 run until you feel satisfied that it is stable. Repeat this process until you find the LOWEST stable voltage at your maximum multiplier and then drop the multiplier to a lower number. Repeat again testing with prime95 until you find the LOWEST stable voltage for each of the multipliers you wish to use.
That's it folks your done, now not only are you getting a top of the line CPU for pennies on the dollar, but your also getting the most battery life you can out of your new powerhouse of a laptop.