It's more or less a capability rating. The 800MHz RAM is rated to provide data to the CPU faster than the 667MHz. However they can both operate at 667MHz or lower or even higher, though if its higher than the RAM is rated for its considered to be overclocked.
No there is not a big difference but if your going to add RAM you want it to be capable of running at the speed your system is currently set. If your system is running RAM at 800MHz already you don't want to have to underclock it to add some 667MHz RAM.