How are you going to tell when the bags are full? what will you do if one bag fills before the others? you should consider some "vent" valves that will take the overflow when the bag gets full. Thats how mine is set up. all I gotta do is turn the pump on, and wait till water starts coming out the side of the boat. Done. I think Craig may have been talking bout Rule pumps. Attwood makes a 1200 gph pump. Its not reversible, but it is only 34.00. http://www.basspro.com/servlet/catalog.TextId?hvarTextId=52431&hvarTarget=search& cmCat=SearchResults
Also, I would suggest a Drainmaster Gate valve. Its actually from an RV sewer system, but they sell one with a remote switch and an LED. If you look around, you can find one for < $100.
Post some pics when you get it installed.