the idea is to have a 4 times turnover of water per hour. (or more... but consider current on fish as well)
so 180 * 4 = 720 gph
but that's 720 gph out of the pipe in the tank, so you have to consider the lost because of height so if you go for a mag drive and we suppose that water has to travel 5' in height, according to the chart
http://www.marinedepot.com/aquarium_...me.asp?CartId=
we know that the 950pgh mag drive will give you 750 gph in your tank.