RAM is supposed to downclock but in the case of older macbooks there are known issues with it. That's why the first response suggested one known workaround of using one slower speed RAM module so the faster one is forced to downclock.
Another known workaround is to use a Windows computer and edit the memory timings so that it will run at the slower speeds.
Buying two faster modules and hoping they'll work is going to be an exercise in frustration. Even if it boots you'll eventually encounter random kernel panics and all sorts of other seemingly random errors. Then again it might work but remember that memory incompatibility is not a valid reason for return, generally.
