That's a great idea! It would probably be a homemade raspberry pi laptop because it's open source. Other laptops aren't but normal laptops have much more RAM.
laptop GPUs are integrated and sometimes even the CPU. The RasPi doesn't quite have the power to run graphic intensive games. You are best off just buying a gaming laptop.
Impossible nowadays, sorry to say. As Fidel says, normal laptops use integrated graphics and the motherboards are custom made for the laptop cases. Desktops are designed for expansion, laptops not so much. Once upon a time companies included PCMCIA card slots in laptops to provide upgradability. You could actually buy PCMCIA graphics cards and install them in your laptop. Then companies figured out that they can make more money if they don't give you those options, (plus they make a laptop bulkier, more expensive, and more power hungry) and instead force you to buy a new laptop every ~3 years. Processor-wise, laptops aren't too bad for games, and you can upgrade your RAM (each model has it's own max supported), but the GPU is a no go.
Considering how much power a gaming PC requires I think its will be a huge task 500w draw is probably the minimum a decent PC setup would require, then there's finding a way of getting 12,-12,5v,-5v,3.3v grounds and keeping the amps high enough so everything is powered enough not to cause issues
As mentioned by several before me, the GPU of a laptop is generally integrated. Everyday cheap laptops are meant for web browseing mostly, and generally do not have a powerful GPU, which means no matter what CPU and RAM you decide to upgrade with, you still won't have the power necessary to run the laptop as a gaming laptop, especially with a lot of new games requiring intensive hardware specifications.
Also, generally for the around the same price as it would cost to upgrade a laptops CPU and RAM, you could build a pretty decent gaming desktop.
Check out "Austin Evans" on Youtube. This guy has a lot of great information and tutorials on technology and building/ upgrading computers. He also has a lot of tutorials on how to build gaming computers with a multitude of different budgets ranging from just a few hundred to a few thousand, so not matter the budget you have in mind, this guy can help you with the information you need to build a decent computer. He also lists all the parts and where to find them within the description of his videos.
