It's just usually not very financially-wise to do so. Usually you can only upgrade to a CPU that has a minuscule performance increase, but it's very expensive. Same with the GPU... And not all form factors for cards are the same; I've looked into upgrading the 5870 in my G73, and there's only a few cards I can upgrade to, and I would have to hardware mod a couple of them (there's several different brands that are possible).
So while it may be possible.. it's usually just not advisable.