The Core Duo and Core 2 Duo are both excellent choices for use in a carpc. The wrapped application architecture (i.e. front end on top of desktop apps) of most of the carpc software we use tightly aligns with the work profile that these chips were designed to excel at. Low power, heat and cost are also compelling arguments.
I'm in the process of upgrading my rig to a T2400 (core duo) based system. I don't have much time these days so the upgrade has been progressing slow but performance of this setup on the bench FAR exceeds my current M10k solution.