I've always liked this article about proper flow.
http://reefsources.itgo.com/guide/pumps.html
This topic is always open for debate. As a baseline, for the most part, the starting point for a tank should be between 10-20 times turnover for total volume. That would mean, for a 50 gallon tank, you should start with 500-1000 GPH (Gallons per hour).
Now, for many people with some experience, this is pretty low amount of flow. Many hobbyists prefer a turn over rate in the 30-60 times total volume. (1500-3000 GPH for a 50gal). This higher rate of flow helps keep detritus from settling on the substrate or the rocks, and in turn, the detritus is picked up by your filter and removed from the tank.
You really should be looking at two or more powerheads. If you use only one, your inhabitants suffer from a boring, unilateral current. (Equivalent to a 20mph wind blowing from the East...every day....24hrs a day...) You can imagine how this can effect growth of corals, should you choose to add them in the future.
Also, having unilateral flow creates "dead spots" in your tank where detritus can/will build up. The buildup of extra nutrients in these "dead spots" will, almost always, promote unwanted algae growth. If you do not want the headache of trying to rid your tank of nusiance algae, it would be smart for you to deal with these issues before they start.
All in all, get a minimum of two powerheads. Total turnover should be around 2000-3000GPH.