The skinny on PAR

[asylumdown]

I know I'm being lazy and that this information is out there in a myriad of places, but maybe if I toss it out to you guys my question can be answered with a single keystroke...

What is a 'good' PAR reading? I hear people talk about fixtures putting out '200 PAR at tank bottom' or '380 PAR at surface level', etc. etc., but what exactly does that mean? What PAR rating is acceptable for what type of coral? Will SPS grow under 200 PAR? Is there a golden number we should be aiming for at certain depths?