you need a skimmer, a return pump, a heater, and preferably a bubble trap. there was a recent thread about bubble traps. if you look it will tell you exactly what that is. some people will also use the sump to keep a UV sterilizer, extra live rock, plants, sand, and a secure area for breeding pods.
like they said, the bigger the better, as long as you can afford it(that's where i have problems ). just make sure you get a sump that's not too tall. to check height requirements place your skimmer under the tank as elevated as possible off the floor accounting for clearance necessary while removing the skimmate cup. then measure from the floor up to the recommended water level of the skimmer. some water will drain to the sump when your pump turns off, so you need to know how much cu. ft. of water will drain. add the determined height of drained water to the height of water during normal use,then you will know the maximum water height and how tall of a sump to buy.