EarthDog is correct, I upgraded my T-30 (see sig) from a P4 - 1.8 to a 2.0 to a 2.5 the fan runs constantly now where as with the 2.0 it would shut down after a few minutes then start up again in a few, now it doesn't shut down. (This will drain the battery faster BTW).
One of the side effects of doing the CPU/Memory upgrade is the FSB went from 400 to 537 (537 not 533 - [shurg]) and I beleve that is the cause of the heat increase.
Check the manufacture's web site for the model number and see what they used as the max CPU/Memory combo's by the end of their production run.
IBM lists the T-30 with a P-4 2.0 and PC2100 memory at the end of the production run but there was a foot note that the motherboard/bios would support a P-4 up to 2.8, I didn't get that coragious even thouth I have a P-4 2.8 M I didn't use it.
Another thing you need to keep in mind is the Video Ram, how much does the laptop have and can the system memory be shared? My T-30 has 32 Meg and it can not share the System memory, thus when I loaded World In Conflict it puked on me, not enough Video ram, needs 128 meg...
Good luck with your project if you go forward with it...