If it's feasible for you, I'd definitely recommend going with intel or amd for your next gpu, due to the whole FOSS drivers/specs thing for one, and yes-- I have had problems with my geforce 8400m gs. I applied a bios update, and it's been stable since, but I haven't stress tested it yet-- I should do that soon. Good old nexuiz
Also, If I recall correctly, the X4500 is /already/ brilliantly supported under Ubuntu Intrepid and such.
I hope centrino 2 isn't expensive just because it's new, I'd certainly go with it-- intel is amazing with all their drivers, none of this broadcom wireless or creative sound garbage.
Heat problems with the 8400m GS? I read info from Notebook forums and supposedly, the 'fix' is a BIOS update. But, a point was made that the defect is hardware and so how is software going to fix it? I think that makes sense. I would not be surprised if Nvidia and the laptop manufacturers have a joint recall and offer to fix the problem by a modification in the computer. Already, on one of the notebook forums, there are posts of people creating their own 'fix' by sticking copper and thermal paste onto the nvidia chip.
I don't like options being limited and doing your own repair is something I wouldn't mind doing after owning the laptop for some number of years. But, I don't think it's something I want to do on a brand new laptop I just bought. That's why I was thinking of waiting for the new generation to come out but I'm concerned about pricing for those.
I don't game but I might want the option of using 2D and 3D graphics (Compiz, Editing photos, Photoshop-like programs, Gimp etc. etc.).