Here are some things to consider:
The CPU in the laptop is Intel® Core™2 Duo Processor P8400 - http://ark.intel.com/Product.aspx?id=35569
The TDP is 25 Watts
The new CPU that you are planning to install is Intel® Core™2 Duo Processor T9900 - http://ark.intel.com/Product.aspx?id=39312
The TDP is 35 Watts.
Come up with a plan to deal with the additional heat produced.
The new CPU may work well at the existing voltage setting. If not, you will have to set the optimum voltage in the BIOS (hope your BIOS will let you change voltages).