Upgrading the CPU on a laptop is far more complicated than doing the same on a desktop PC. With a laptop it's important to buy as much performance as you can afford, even if you think you don't need it at the time, because apart from the hard drive and RAM, you are pretty much going to be stuck with what it's got.
I found this from one website, and it more or less confirms what I've said:
"The more complicated question for upgrading laptops is CPU replacement. CPU failure is fairly rare even under overheating conditions since modern CPU's can protect themselves by shutting down on thermal overload. But if you're looking for a performance gain by upgrading your laptop CPU, you're likely to be dissapointed. Many laptops employ surface mount CPUs soldered to the motherboad, which are non-replaceable. Some laptop motherboards will support a whole series of CPU's from Intel or AMD as long as they use the identical socket and voltages, but it may require a BIOS upgrade for the system to recognize the upgrade CPU at the it's full speed. Flashing the BIOS of your laptop unless you absolutely have to is a bad idea, an error in the process or a mistake in downloading the correct BIOS upgrade will leave you with a brick. Furthermore, the performance gain in upgrading the CPU with a chip that's 20% faster will yield dissapointing results, as the speed increase will only be apparent in certain computation intensive tasks. Finally, the new CPU may dissipate more heat than the original, and the heatsink/cooling capacity of your laptop model may not be up to the task, even if the motherboard supports the speed. And higher power consumption also means a shorter battery life".