If you bought your RAM at a place that also sells PC RAM you may have gotten mislabelled RAM. PCs can handle different RAM speeds so they’ll often label higher speed RAM as a lower speed rather than make two different speeds of RAM. But Macs are much more picky. They require a RAM stick to be exactly 667mhz not 675mhz or 800mhz. The way to tell is to put one of your old RAM sticks in and it will force the new RAM to run at the correct speed.
This is from one review of PNY RAM:
"These modules are actually 800Mhz. PNY no longer makes or sells 667Mhz modules. Not all computers that require 667Mhz are compatible with 2 800Mhz modules. They refuse to down clock properly. This is especially true with a number of Core 2 Duo MacBooks. Spoke to PNY support, they flat out told me that yes, they sell 800Mhz modules in 667Mhz packaging. If you RMA a module that is 667Mhz (or supposed to be 667Mhz) they will replace it with an 800Mhz module as they no longer have any 667Mhz SODIMMs, not even for RMA replacement!"
These are good online stores for Mac compatible RAM
OWC http://eshop.macsales.com/shop/memory/MacBook/DDR2/ - They offer Mac tested RAM at very good prices.
Crucial Memory http://www.crucial.com/ - good place to buy RAM from all over the world. They also have an excellent memory selector that allows you to choose memory based on your computer's model
Data Memory Systems http://www.datamemorysystems.com/apple-memory.asp - another good, cheap place to buying RAM if you live in the U.S.