Originally Posted by
Biomega
It's very hard, yes. Most laptops are not made to be modular, save for specific elements. A lot of components are fixed in place and very difficult for the average user to replace, let alone assemble. It's not impossible, but without extensive technical knowledge and access to specialized components, I wouldn't even TRY to do this. Given that you're here asking for advice, it's likely that doesn't apply to you.
Not to mention that any laptop will be significantly more expensive than a desktop for comparable power, while running into serious limitations the higher up in performance you go (like cooling, power consumption, etc.). You can easily make a monster desktop for the same money you'd pay to just get a reasonably good laptop.