RAM comes in the quality batches much like cars:
Cheap - budget
meduim - reliable
& High - Lifetime warranty.
Crucial.com and Apple both sell High end ram that is guarenteed to work with your system.
Other vendors: Chipmerchant.com, Macsales.com, etc... sell medium budget friendly RAM that has worked fine for me in my Powerbook for 3 years.
Newegg.com usually sells the Cheap stuff that will likeyly give you some type of problem and wont last. Kingston RAM is pretty notorius for not working in MACs althou sometimes they do, if you get them through a reliable reseller.
The price & Reseller is the best way to determine Reliablity.