I don't want you to downsize the cpu, it's just that the 2400+, 2500+ etc. are all about the same. The multiplier isn't locked with the mobile cpu's. They'll easily overclock to 3000+ like these extreme examples show.
http://forum.lowyat.net/lofiversion/...hp/t84965.html
but I'd advise you to run it at a lower speed. At 2800+ and 1.45 volts or something like that it should still run much cooler than the sempron would at that same speed (running at 1.6 volts).