There is a law:
http://en.wikipedia.org/wiki/Moore%27s_lawBased on that:
You don't buy pc or pc parts if they are not at least 2x better than your current.
So, it's a regular behaviour to change your pc every 2 years. With the laptops , this rule is even bigger, because most of the laptops don't have a good durability and after of 2 years of heavy usage they are literally falling apart.
And when the 2 years passed away:
9800gt --> 560ti sonic
4GB at 800MHz --> 4GB at 1600 MHz
dualcore at 2.5GHz --> quadcore at 4.5GHz