Purchase with cheapest CPU you can get.
Then go to ebay and pick up a 1.6GHz 100/400 MHz P-M (7x5 series).
Pin mod the socket so it stays at 133/533 MHz
(it's a simply jumper wire plug in, no soldering).
End up with 2.1 GHz for cheap. (if you really feel brave, you might get 2.4GHz+)
Since you asked about overclocking, then this is nothing to be scared of, as you are probably familiar with overclocking desktops.
Huge threads at notebookforums.com explaining this all on that laptop.
The nice thing about these P-M's is how COLD they run.
Many have run their overclocked P-M's at 2.0GHz in a desktop with NO COOLING fan!!
http://www2.ijib.com:1337/phpbb/viewtopic.php?t=3
http://www.pcper.com/article.php?aid=133&type=expert&pid=6
Look at these temps with no fan:
http://www.madshrimps.be/?action=getarticle&number=5&artpage=1155&articID=289
With these P-M's underVOLTING is the thing to do.
When undervolted, normal stock clocked CPU will rarely use fans, and this along with reduced CPU vcore, will greatly extend battery life. Do gaming on A/C with full power and full voltage, then drop to speedswitched speed and low vcore when on battery power.