I'd second crucial. It is the same quality RAM but for much less than Apple. I have a 512 stick in my 12" PB from crucial and have never had a problem.
i think newegg.com has some good prices for ram. i got a kingmax 2100 (512 meg) stick for my 1GHz 12". it gets good user reviews. so far it's been good for me. it's about $84 now. i don't know what speed the new 12" ram is. i think the 2700 version is a dollar more.
crucial ram from newegg is also generally cheaper than direct from crucial.
They all have good RAM. I've used RAM from 4allmemory and coastmemory and have never had a problem with their RAM. A buddy of mine always uses RamJet RAM without and trouble.
Some are cheaper than others...you just have to look around.