So, how do you make sure that you have enough laptop for your needs? Let's start off with some base guidelines, and work up from their to see how much you need. First of all, if you're going to be running Window's XP (which you most likely do), you'll want to get a computer with at least 512 megabytes of RAM. The same applies to Apple OS X. If you plan on installing a lightweight Linux distribution on your laptop, then you can go with a little less, but more is always better.

As for the processor, most anything above 1 Ghz will work, though not at extremely high levels of performance. Stay away from the Celeron processors, though,as these tend to be much slower than you would expect for their given clockspeed. Also, keep in mind that some older processor (Pentium 4s in particular) would burn through the battery quite fast. In these cases, you might be better going with either an older, cooler processor, or a newer, high efficiency model.

Screens have also improved with age. Try to find a screen that has a resolution of 1024x768, at the very least. Anything lower than this will be useless to you. Also, make sure that you're getting an adequately sized screen for your needs.

When it comes to other parts of the computer, things will really start depending on your individual needs. You'll probably want at least a CD drive, though many will also want a DVD drive. Gamers will need some sort of onboard, dedicated graphics to play even the most rudimentary 3D games well. Mobile users who want internet should look for built in wi-fi, though if this isn't available, you can usually buy a PC card wi-fi adapter that will do the job quite well.

So, this should give you a good base line to go off of when looking for a used notebook computer. You may not end up with the flashiest computer out there, but if you're needs are mainly constrained to internet/e-mail/office style tasks, you can get away with a much older used notebook computer then you might otherwise be able to.