so,
would a nvidia card be better/safer choice for a linux-notebook?
for drivers the answer is yes. nvidia has the best linux driver support right now. the geforce4 in the laptops is equivalent to a geforce4MX not a Ti model so they don't have any of the advanced hardware shaders. the ati 9000 has hardware shaders.
GEM is not using the hardware shaders yet... but i have an ati 9000 in my laptop (g4 titanium) so the possibility is there to write some stuff. however, it is probably best to wait until openGL 2.0 and it's unified HSL (hardware shader language) is available in final form. perhaps by this time next year most of GEM's image processing can be done fully on a chip like the ati 9000. the geforce4mx will never be able to do this sort of processing (it's actually closer to a geforce2mx than geforce4 ti).
so your choice is basically between compatibility and driver support today, and the possibility of high performance processing in the future. if you must go linux then nvidia is probably the safe choice, but the ati chip will be better in the long run in windows and possibly linux depending on the driver support. all driver issues being equal i would choose the ati 9000 over the nvidia geforce4mx without hesitation.
oh and 3.7kg+42mm thick with a desktop P4 doesn't sound like a very nice portable device. hot, heavy, and battery hungry ;)
cgc
i was also thinking on one of those toshiba satellites with gforce4... but the design of gericoms masterpiece is much cooler... (i just love those round edges :)
cheers -andre