I second the recommendation for Crucial and OWC. I also use OWC for most of my HD purchases (internal and external).
Word to the wise though - both offer lifetime warranties on RAM. OWC is usually cheaper, but I've also sent back several OWC chips. It has never been a problem and they've always promptly replaced the RAM that went bad, but it is something to keep in mind.
That being said, I still typically use OWC for my RAM.