Go with a good brand over the lowest price you can dig up. We got our 512MB upgrade through
Kingston directly, but url=http://www.crucial.com/]Crucial[/url] is good too. The best part is that they guarantee compatibility with the computer you specify, so if something doesn't work, they'll make it right.
There are a bunch of other brands, but I've stuck with these two because they have made it easy to shop with them. Go to either site and use their configuration tool; it will show you what you need for your particular model.