NB -> North Bridge -> Memory Controller
This is not the same as your memory frequency. It sounds like you have an AMD rig. If so, you likely are running your memory at 1333 (or with some possibility of 1600). If you add the 1333 kit to this mix, you will HAVE to run the memory sticks at 1333. No choice on that. Also, since you have 2 sticks per channel, the Command Rate will be automatically set to 2 instead of 1.
If none of that sounds like an issue for you, then I'd say it is safe to do so. Although if you have not purchased the memory yet, then maybe you can consider 1600 ram, since it is not much more $$$ and you can OC a bit later on with the flexibility of memory speeds.