hi Julien,
my (and others') problems with mobile Pentium4 processors are very well-documented on this list! I simply cannot recommend them for *any* kind of realtime audio processing, especially under Linux! This is due to the "denormal numbers" bug in their architecture. When the mantissa of a floating point number gets X number of places below the decimal point, it kicks into "extremely accurate mode", and there goes your DSP. Calculating the tail signal of a reverb (very very small, almost infinitely quiet sample level) is a classic situation where this occurs. Later generation P4s may have some fixes around this, but the problem still exists.
I got rid of my last P4 latop and got a Athlon64 because if this. If you don't mind a hot, heavy, loud computer (I could blowdry my hair in the morning with mine!) that lasts about an hour (in my case) then you might investigate this. For me, besides the fact that I put an extra kink in my spine every time I go on tour with this machine from carrying it around, the biggest issue now isn't CPU power. I can't remember the last time I hit bottom with this laptop. It's the noise, and all those desktop CPUs in a small casings will have this problem. So doing any recording work with microphones in the same room is completely out!
good luck, d.