How old are you? More importantly, how close are you to getting into college? If you're a ways away from going to college, your parents may be able to afford a new computer right now and save up for college as time goes by. But if you're close to going to college (like, in high school), I suggest you wait until you graduate and get a good job to get a nice, expensive computer. You should put as much money as you can toward getting a good education because, unless your parents are filthy stinkin' rich, you're probably going to have to get student loans, and you'll have to pay those off for many years after graduating, not to mention a new computer.
Depending on what your needs are, you could get a cheaper computer now and save up for a nice expensive machine later. You can get an eMac for $749 through the educational store, which would take practically no time to pay off (payments are $7/month). Sure, it's not a G5 speed machine, but it's still quite a decent computer.