Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
As other people have said, I'll add myself to the list of people recommending OWC RAM from Macsales.com

The price is right, the reputation and lifetime guarantee are incredible.
 
Replacing RAM and the hard drive in a Macbook is considered "user upgradeable," therefor your warranty will be fine still unless you tear it up by replacing it.
The best places I have found to go for RAM is crucial and macsales.

If all else fails, you can always find good RAM for good prices on eBay.

Just make sure you know what you are buying before you buy it. Different speeds are compatible with different computers. Your RAM is more than likely PC5300S DDR2 SDRAM @ 667MHz. Macsales and crucial will tell you what is compatible, on eBay, you have to know your stuff or else it's going to do you no good.
 
The right question is "Is buying RAM from Apple worth it?" The answer is: NO! They mark their RAM up by 4. Seriously. 3rd party RAM is officially certified to work. I have a 1st gen MacBook. I bought new memory from OWC. The 1G memory sticks I got from OWC are the same exact manufacturer as the 512M chips I pulled out of my MacBook. And they were a quarter of the price that Apple was charging.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.