I recommend http://www.ramjet.com
they are competitive, reputable, with excellent installation instructions online. RAM is a good bang-for-buck investment.
I've bought from them three times (iMac Indigo in 2000, iBook in May 2002, iMac G4 in in October 2002) and I'm a satisfied customer, but, where are those instructions?
www.18004memory.com These guys have the absolute lowest prices and they beat any price also. They also garrantee that the RAM the ship you will work with your Mac. Lifetime warranty and all of that good stuff. I only buy RAM from these guys.
These guy are the very best, plus the memory they sell is Apple brand, it cost the same as kingston. Plus price is very low, I looked a the other memory vendors listed here and memoryx sell for the same price. Plus they come with the deal of warrenties etc etc etc etc...