That is correct. You need a Pentium M Dothan 533Mhz FSB processor.
Upgrades for you will be
1.73Ghz 533FSB 2MB L2 Cache - 740
1.86Ghz 533FSB 2MB L2 Cache - 750
2.00GHz 533FSB 2MB L2 Cache - 760
2.13GHz 533FSB 2MB L2 Cache - 770
2.26GHz 533FSB 2MB L2 Cache - 780
Good luck finding them though, they are getting rare. Might want to hit up ebay.
Here is a list of ALL the CPUs you can use in your laptop, but only the ones with the 533Mhz 7x0 series chips above 1.6Ghz will give you an increase in speed.http://www.intel.com/products/proces.../pentium_m.htm
All of the 7x5 series chips are the pin moddable CPUs (those are the ones rated at 400Mhz FSB)
They pinmod to the following speeds.
1.6Ghz --> 2.13Ghz
1.7Ghz --> 2.26Ghz
1.8Ghz --> 2.40Ghz
The 1.6Ghz chip has about a 90% success rate.
The 1.7Ghz chip has about a 50% success rate.
The 1.8Ghz chip has about a 30% success rate. (rare)
The 2.0Ghz and 2.1Ghz chip(s) produce so much heat, it's nearly impossible to get one that will pinmod without heavy modifications.
Pinmodding a CPU is pretty simple, just takes a little time. You have to find a very, very thin piece of wire and cross two pins on the CPU socket, then put the CPU in and it's pinmodded. Basically, it's just tricking the 400FSB CPU into running at 533Mhz FSB speeds, thus overclocking it in the process.