Beki Beshi

べき as in するべき, something you 'must do'. Recently I learnt the term べし which seems to be used in the same context as べき. Are they essentially the same in terms of usage? Any difference in politeness?

3rd August 2005 Update:
べし is like べき but old style language. Also, people use すべき・すべし for するべき・するべし in conversation, though I'm not sure if they'd be written like this.