How long should a person wait before purchasing a new computer?

I'm thinking of getting a new computer before I head to college, however the computer I'm using is only about three years old. Money isn't "tight" for me but my wallet isn't exactly burning a hole in my pocket either.

How much money should one be willing to spend on a computer if they need graphic cards that can "handle" things but don't necessarily need them to "exceed" requirements of most graphic utilities?

I suppose it is a silly question, if I want a new computer and can afford I suppose I should go for it, but also I hate spending money, so I don't like to do things unless I think it is important.

Also what if I need money for something actually important later X.X. I hate this concept, I know as soon as I waste the money either I, or someone else I care about, will need money.