I do all of my computer shopping through Newegg. RAM, hard drives, anything computer related they have. I've probably spent over 3000 dollars there no problem. They have really good prices on 3 day shipping and I've never had any issues arise with my orders. Also from what I've heard they have a really good customer support team as well.
OWC. Guaranteed for life, and they're Mac people, too. They even have a trade-in program for your old RAM, although with prices being what they are, it's ain't much.