Someone once told me that if I put non apple ram into my computer that it would void my warranty. Is this true?

Only if Apple can prove that the non-OEM part caused the failure. Federal consumer protection law(s).

Rarely (as in I've never seen or heard of it!) an issue - you're fine.

What i would do is if you ever have a service call, just pop the old memory back in. Then you would be fine. But most of the apple techs are pretty cool, we deal with them about once a month at my day job where i run about 40 macs.

It will not void your warranty. Just make sure and get good RAM - http://www.transintl.com has some of the best prices and best support for apple RAM - the cost is about 1/4 to 1/3 of what you'd pay getting it from apple.

I have third-party RAM in my G5, MacBook pro and Mac Pro and all have seen applecare for various (small) reasons and the RAM has never been questioned. They just don't cover your RAM obviously - but transintl (and a few other memory resources) carry a full lifetime warranty on the memory that they sell