It's been a long time since I've worked with amps, but my understanding is that the wattage rating is the maximum available output, not the nominal output. I'm reasonably certain that your PA system will simply draw what it needs from the amp, and the amp will supply what is demanded of it up to it's rated maximum output. So in your case, your 100 watt PA system will draw 100 watts of power from your 200 watt amp, and the other 100 watts of amp capacity will simply remain as unused available overhead.
Just did a little googling and sure enough:
http://www.chasehometheater.com/forum/s ... .php?t=190So you should be fine with that big honkin' amp. Just make sure that the impedance (Ohm rating) matches.