Whether it's a good idea for you to build your own computer depends on a few things.

1. You know the things that are inside a computer.
2. You are good at bargain hunting.
3. You have the patience to read a good variety of reviews for every part that is important.

If these conditions are met, you can build yourself a $400 dollar computer in 2006, that still plays all modern games at smooth medium graphics level. But the process will take you many hours of surfing to find the parts you want, and many weeks of waiting for the right sales.