Techradar posted a tutorial about installing Mac memory

Unless you put the maximum RAM in your Mac when you bought it, chances are you can add more, which is what we're going to help you do. Remember, new memory is an affordable way to get your Mac running faster. Depending on what Mac you've got, you may need to take out the existing RAM to put more in, so bear this in mind when you're buying. If your MacBook Pro already has two 1GB chips (one in each slot) and you want to upgrade to 4GB, you can't simply buy an additional 2GB. Check System Profiler for what you've got now. The best place to get memory from is Crucial, which has a simple guide to ensure you buy the right RAM for your Mac.

  Tutorial: How to install Mac memory