Pump suggestions needed

I am working on putting together a 54G Corner tank for my next aquarium.  I am looking for suggestions on pump sizes.  The tank has the built in overflow.  I am going to do a closed loop on it as well.  How big of a pump should I use for the sump?  I am thinking about doing a dual direction closed loop and using alternating pumps.  So one pump would push water one direction for a time, then turn off, then the other pump would kick on and push water the opposite direction, and back and forth and so on.  Any suggestions on how much flow I should need for each of these pumps?