Another thing I'd add to Joe's general flow comments, which he alluded to, but might not have been totally clear, is that wider flow patterns are better than narrow high-velocity flow patterns.
If a pump can push out 1000 gph of flow, and you put a 1/2" nozzle on it, you'll have a jet of water flow that could cut steel, and strip tissue off of flow-loving corals. But if you move that same 1000 gph through a 3" nozzle, you'll have a very effective flow that moves across much of the tank, and affects a broader area, but without the damaging force of the narrow jet.
Another advantage of this broad flow pattern is that it generally consumes less energy than forcing the same amount of water through a restricted nozzle, so you can get more flow-per-watt, which is always nice.
This wide-pattern is the idea behind the maxijet mods (changing them from 1/2" to 3" outlets), tunze pumps, seios, etc.
I'd say for powerhead flow these are really the way to go.
I'd also point you to Eductor nozzles. They are an amazing way to increase flow and improve the flow pattern from large external pumps. There's an article on them in the "Useful Threads" sticky at the top of the ReefTalk forum.
Nate