It all depends on what you are using your computer for. If you are just surfing the web, downloading , and simple duties, leave your comp alone. It does those jobs efficently and a faster cpu/mobo/psu would not benefit you.
If you are a gamer, multi-tasker, or a audio or video editor, then these upgrades would be more useful.
To be perfectly honest with you, many people upgrade their computers not because there old ones dont work, but they want to have the most recent hardware. In the field of computers however, technology allows for many quick changes. You can buy a dual core cpu now and it is already going to become outdated within a year because of quad core. There are numerous other examples of this.
Therefore, I would analyze what you use your comp for. I thought I needed to stay up to date on my hardware, and upgraded once a year. Now I realize that it is a waste of money, unless you are made of it. The old hardware works fine and dandy, but this newer innovation that just came out just works a little better. Personally, I would try an upgrade maybe every two years or so.
I know this is a computer forum and many of the members will not agree with me on this, but just think about your actual needs and dont buy a new peice of hardware because of what it can do if you are not going to ultilize it.