Yes, you can....within limits that are akin to desktop systems.
First off, for most mass market laptops, this answer is no. Because of economies of scale, they design the PSU and cooling sub systems to the hairs breadth of design limits and no fiddling after that as something will get overloaded. Then there's the boutique industry which custom builds and these designs need the flexibility to handle those changes. It's a little harder now to figure out who makes what but you could go on sites like Falcon northwest, WidowPC etc and notice that all the models kinda look the same.... two year's ago they were all used Clevo as the ODM but most builders are now starting to use other ODM manufacturers.
To learn about Clevo, look here:
http/forum.notebookreview.com/showthread.php?t=91510
The most common gaming laptops are based upon the Clevo 860CU and 870CU designs
http/www.pro-star.com/index.cfm?mainpage=product&filter=6
GFX card choices are:
NVIDIA GeForce GTX 280M w/1GB DDR3
ATi Mobility Radeon HD 5870 w/1GB GDDR5 DX11
NVIDIA GeForce GTX 285M w/1GB GDDR3
You'll note that there is not a large difference in performance between the 3 choices. The base model comes with the 5870, with a $45 discount for the 280M and a $55 increase for the 285M