It's quite easy to upgrade your MacBook's RAM. Your warranty will not be void. The two parts that users can replace without a total dismantle, thus triggering any warranty issue; are the RAM and the battery.

Learn how to correctly install memory into a MacBook portable computer.
Your MacBook computer has two memory slots that you access by removing the memory door in the battery bay. Your MacBook comes with at least 512 megabytes (MB) of 667 MHz Double Data Rate (DDR2) Synchronous Dynamic Random-Access Memory (SDRAM) installed.
Both memory slots can accept an SDRAM module that meets the following specifications:
* Double Data Rate Small Outline Dual Inline Memory Module (DDR SO-DIMM)
format
* 1.25 inch or smaller
* 512 MB or 1 gigabyte (GB)
* 200-pin
*
PC2-5300 DDR2 667 MHz Type RAM - Very Important!
Follow the steps in the link below.
http://docs.info.apple.com/article.html?artnum=303721