A new computer is always going to be better and faster that what ever you have now. Whenever you should buy a new computer depends on what you really need. If you primarily do email and web surfing an older computer will do fine. If you are a heavy duty gamer, you will need to either buy a new computer frequently, or upgrade often.
I remember a cartoon several years ago. A customer was buying a computer at a check out counter, and at the end of the check out counter was a trash can, meaning by the time you paid for the computer it was already out of date.
