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.
I got it for $28.xx last week, now rebate went down from $20 to $15 per stick but it's still a very good deal: (2x$48.99 + ~$6 shipping) - 2x$15MIR = ~$75 for 4GB DDR2-667 laptop memory. 😎
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.