First decide if you want Mac or a PC. Remember that Macs have a different operating system so if you're used to using Windows that you'll either need to learn a new operating system or you will need to buy Windows ($150+) and add it to your Mac if you don't want to learn a new system. You will also want to take a look at all the software (editing tools, etc.) and hardware (printers, scanners, etc.) you use and make sure that it is compatible with the system you want to use or determine if you would need to purchase new.
Personally, I decide what computer to buy based off of what I'm using it for. If I were to do a lot of graphic design, major photo editing, etc. then I would go for the cheapest Mac. Since only when day dreaming do I do a lot of those things I work on a PC. The "gap" between Macs and PCs in quality has narrowed quite a bit in the last few years. In fact, Macs often use several of the same parts that you find on much cheaper laptops.
Cnet has a very good article that outlines the different laptop users and a possible ideal set up for those users: http://reviews.cnet.com/2719-7602_7-...?tag=page;page
Finally what I have is simply one of the cheaper laptops you can pick up pretty much anywhere. I figure $300 every few years is how much I would end up spend servicing, upgrading, etc anyway so I might as well buy a new one when my current no longer functions as I wish it. My current super cheap laptop (Acer) is going two and a half years strong with no major performance issues... now only if my desk top (which is of MUCH higher quality and custom built) could boast the same.
Good luck finding the right computer for you. Just remember, pick what will work for you because you're the one who will use it!