Will not boot after memory upgrade

I have a 13" early 2008 Macbook and have just purchased 4GB (2x2GB) DDR2 800MHz Corsair memory modules. The machine will not boot when I install both memory modules. However, installing either of the modules with one of the old 1GB (667MHz) memories boots fine. What is the issue? Aren't 800MHz modules supposed to be backward compatible?