If you are running 10.6.8 do you actually have any warranty to void? Replacing RAM yourself would not void a warranty. It only precludes any damage you might do during the process from being covered under the warranty.
If you take your computer to an Apple retailer they will upgrade RAM for you. If you want to do it yourself it's pretty easy. Visit OWC to find out what RAM you should use, tutorials on how to install it, and any tools you may need (actually just a small Phillips screwdriver.)
If you open your user manual you will also find this information in the section Adding Memory.