There's been several threads posted lately about this topic, so I think a sticky of this would be helpful.
1) Can I upgrade the CPU in my laptop?
--> The answer is most likely no. Laptops are built to tight thermal and power constraints. Also, like prefab desktops, they use customized motherboards with customized BIOSs which often have support only for a couple of processors. Which ones are supported can be found out by asking whoever made your laptop.
2) Can I upgrade the GPU in my laptop?
--> The answer is most likely no, mostly for the thermal and power constraints.
3) What if I'm a genius and hack in support for more processors in the BIOS and add in additional cooling?
--> Then you wouldn't be reading this post.
4) Ok, so I've contacted my OEM and they gave me a list of processors my laptop supports, and I can find one on eBay. Should I upgrade?
--> Well, it depends. 10% faster won't really be noticeable, start getting towards 20% and you will see something, however that much faster and you usually end up getting into new thermal envelopes.
5) But I can get a 200mhz faster processor for $20, you're saying I shouldn't do it?
--> Depends, I can think of a lot worse things to spend $20 on, such as 3 cups of Starbucks flappacrapachino.
6) So I've decided to upgrade it, what do I need?
--> A screwdriver, new thermal material (Arctic Silver 5 and IC Diamond are two popular mixes), a cloth and cleaning material to remove the old thermal material (Arcticlean works wonders, lacking that 91% alcohol, lacking that 70% alcohol), lacking that then get in a car and drive to a place called Wal-Mart or Dollar General and buy some), a compressed air duster to accelerate the drying process and to clean out the internals of your laptop while you have it open, and to read some of the articles on how to apply thermal material correctly.
7) So, I heard back from my OEM and no other processors are supported, or I can't find a processor faster than mine for a good price or at all, how do I squeeze more performance out of my laptop?
--> First thing would be a SSD, compared to the stock 5400RPM drive of a laptop it'll slash boot and loading times significantly. You can also upgrade your RAM to faster and higher capacity models if your board supports it.
8) But I've got a bottom shelf basic model, an SSD costs as much as it did, should I do it?
--> Actually...Yes. Why? Because it can be moved to a new laptop when you get one.
Crosshair IV Formula, Phenom II x4 965 w/ Corsair H60, 8 GiB OCZ Platinum DDR3 1600, XFX HD 7970 Ghz, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, HP ZR2440w, Win 7 Ultimate x64