First things first, be careful. Don't do it on the driest, most static electrified day of the year. Somethings going to pop on you.
No matter what model Mac you own, make sure that all of the RAM is from the same manufacturer and all the same size. This is a best practice for upgrading your RAM and I've never had a problem with a machine running RAM from one manufacturer.
I have, on the other hand, experienced many times over a Mac running poorly or not running at all when RAM sizes or manufacturers were mixed.
Want to max-out your new MacBook Pro with 4GB of RAM? Buy 4GB of RAM from a reputable manufacturer. Keep the RAM that came with the computer safely tucked away for an emergency.
Erik