Installing third party ram does not void the warranty, despite conventional wisdom. There is a section in each manual for every mac that teaches you how to properly install RAM as such, memory is classified under 'user installable'.
I will second alucard's suggestion of going to SGL. They have the best prices and best service, they're the folks we macheads go to (instead of apple stores), even for RAM.
espn: I wouldn't say 'picky'. More like you can't just go pick up some funny old memory lying around from your old PC, but if you got the correct thing and the correct clock speed, and it's from a reputable brand, then it's fine. Don't tell anyone but even the RAM that Apple gives you as an 'upgrade' is really a third party brand (at least at one certain flagship apple centre I know of..). But at a premium.
Have fun! Anything else you need to ask, it's probably been covered at the Mac forum.