Bitstamp: What type of order to place to buy BTC when the price drops to a certain value?
I figured this out. A "Limit" order is what I wanted to do.

A limit buy order will trigger if the price hits the target or lower. A caveat for anyone unfamiliar like I was: the order won't necessarily be fulfilled at the price your specify, rather it will become a market order which will execute at the next best available price.