Replying to Upgrading for editing standards

So I use a Mac all the time, and it is an iBook. It runs great and I use it all the time but I don't want to put a big editing program on it like final cut just because I use it for everything else and it is a laptop. So my big question is rather than buying a whole new computer would it be worth it to upgrade one I have around my house to run premier on? I have a Dell Dimension 4400 (not sure how fast it is or w/e) So what would be substancial parts to have in the computer to just run Premier and explorer? And a price on the parts it would need.