To be honest mate, if you are 18, and don't have any credit history, don't get in to debt just to get a new computer.
On ebay you can get a used G4 emac for less than £100 - get a 1.25 or 1.42 ghz one, 17" Monitor, Super Drive, 80gig hard drive. Max out the Ram (you can get 2gigs on those things despite apple 'supporting' only 1 officially).
For that you'll have a great little machine, that will run Tiger easily, or Leopard if you really want to push it. It will also handle every day tasks admirably. And all for the price of a couple of nights out on the town.
Edit - Sorry if that sounded preachy - its totally meant as my opinion! If you DO get the new machine its going to be a lot faster and last you much longer. It will be faster at almost everything from photo and video editing, ripping CD's, converting video. Plus you will be able to play games on it that won't run on a G4.
If you just want it to go online, manage your itunes and type a few documents then an old model will suit you fine too.