My Mac desktop came with minimum RAM so I decided to upgrade it. And our iPhone 4's are over two years old and could barely hold a charge for a whole day.
The RAM upgrade was a breeze, only took a few minutes. The iPhone battery was fairly easy, only took 25 mins...5 of that looking for the tiny screws I dropped.
iFitIt makes the repairs really easy. The app and web site make it easy to find the correct parts. They offer the tools needed, and the repair guides are very thorough and easy to follow.
Take your time and read/watch each step (and the next) before proceeding. Work in an area with good light and a smooth, white surface to help find those tiny screws when you drop them.