Upgrading Laptop CPU

I have a Dell Inspiron 2650 running a 1.5MGhz Celeron processor and would like to upgrade to at least a 1.4MHz P4M processor. Is this possible (I called Dell and they said I can go up to a 1.8GHz) but I don’t know if I can get just any 1.8GHz Mobile P4 will do or if I can go higher and whether or not I will need to make structrual adjustments to the laptop chassis to compensate for ventilation etc. If anyone know of specifics please let me know. I am getting a great deal 2.2GHz and would really like to upgrade.