Note: The following is taken verbatim from an e-mail I sent a relative who was interested in purchasing a laptop. It's reposted here so I can share it with the world and subsequently link to it the next time I'm asked about it. That and some feedback from the public at large couldn't hurt!
The best way I found to shop for laptops is to find as many flyers as possible and cut them apart so you have playing-card sized pieces with the picture of the laptop, its price and its specs. The Future Shop and Best Buy flyers are great for this and sometimes the Dell, The Source and Staples flyers are useful too.
You then start to whittle it down by eliminating obviously too-expensive laptops and too-low end laptops. (i.e. those in the $3000 range and those with less than 2 GB of RAM) You just toss the pieces in the garbage. For the next iteration, you start looking more carefully at the specifications, pictures, etc. and tossing out those that don't fit your wants and needs. Repeat until you have about two or three left.
What you do next is you go to the stores, find where the two or three you have left are, and play with them for 5-10 minutes to see if there's a clear winner. If you can't tell the difference between the them, well, the least expensive wins. I recommend you sleep on it at this point.
Laptops usually come with a one-year warranty and you can purchase a few more years after that for up to 3 or 4, depending on the store. That can easily add $300 to the cost of the laptop, so you may want to decide on that before you even step into the store, otherwise you'll be asked to decide at check-out and making a decision about something that's almost 1/3 the cost of the product is not something you want to do quickly.
I would not recommend purchasing a laptop (over a desktop) unless the computer needs to be mobile and moved around a lot. You'll pay more for the privilege of having small pieces that fit inside a portable package and thus the same money could be spent on something more powerful with a bigger screen, etc.