My iPhone 3gs has always shut down randomly and it's battery could hold up to about an hour.
Most of the time it shuts down when I'm surfing the web, taking photos etc..
I found out many people had this problem and the solution is to replace the battery.
So I have a few questions.
Has anyone dealt with this problem before? Did you fix it by replacing the battery?

I read from someone that some batteries aren't good for use, they can cause iTunes errors, freeze your iPhone etc..
He told me that he replaced his battery about three times and he could not restore his iPhone until he bought a genuine battery for about 16$, how am I supposed to know which battery to buy?

Any help is appreciated