You should really look for an all-around brim, if you're looking for protection from the sun. It's not just the top of your head. The most over looked places to protect include your nose, forehead, sides and back of the neck, and especially the tops of your ears.

I like those Boonie hats myself. But, when it's hot and I'm working in the yard, I've got a nice wide straw hat that works well.