Ok, partially answered my own question.
There's a ready made board that can be used directly with LED ribbons:
http://arduino-info.wikispaces.com/Brick-4ChannelPowerFetSwitch
This one is rated for 200,000Hz switching capable -- perfect for my needs.
http://www.elechouse.com/elechouse/index.php?main_page=product_info&cPath=&products_id=1812
They are $18 each for 4 channels (10 amps per channel, 16 amps total), and can switch
On the other hand, if I am controlling 16 ribbons, the costs of these boards add up rather quickly. But the power and safety features (optoelectrically isolated) is extremely attractive.