My laptop currently has an Intel Core 2 Duo mobile processor (T6400) and I want to upgrade it to a more powerful processor. What I would like to know is whether the current processor's 800Mhz FSB or 35W TDP or 45nm or package size or any other specs have to be same with that of the new processor I purchase. And also am I restricted to the same processor family if I intend to change just the processor in the laptop and nothing else?
Some of those things you mentioned are important, but the way to go about it is to find out whcih CPU are compatible with your motherboard so you can:
A) Contact the Laptop manufacturer and ask which CPUS are compatible.
B) look at teh same model laptop sold by the manufactur, but the higher end model and see what CPUS they have.
C) find out what the model and manufacturer of the motehrboard is and contact them.
D) Do some googling and find out what CPUs other people have upgraded the CPU in the same model laptop to.