You need a compatibility guarantee. Apple has some pretty high standards. Bad RAM acts exaactly like a corrupted system file: kernal panics, etc.
But most of the usual suspects are fine. I like to buy from
OtherWorldComputing
http://www.otherworldcomputing.com/
MacSolutions
http://www.macsolutions.com/
DealMac
http://www.dealmac.com
Last time around, these guys had the best price:
RamJet
http://www.ramjet.com