is an image of the design of the wet dry filter which I created..(this is not my own one but will give you an idea).
Basically you have a tower which holds bioballs or dollar store plastic pot scrubbers, above the water line in the sump, water drains down from the main tank via an overflow (see video [ame="http://www.youtube.com/watch?v=65yVr7DiDls"]here[/ame]) into a prefilter, from there it goes into a drip plate (plate full of holes) to evenly distribute water over the bioballs...from there it goes through another filter sponge before being sent back to the main tank with a return pump.
I have an overflow capable of doing 600gph made from 1" pvc tubing, my return pump is 750gph as the water has to be pumped up 5 feet from the sump into the main tank.. I use 1/2" pvc for the return line and a ball valve to control flow.
A sump serves to increase the total volume in your system and make the water parameters more stable, if using bioballs or pot scrubbers (MUST BE PLASTIC ONES) you are creating a massive biological filter in which to house all the good bacteria needed to break down waste. They can be quite expensive to setup as you either DIY a tank or buy a readymade tank or container.
There are literally hundreds of ways to create an effective one and simple google search for "wet dry filter" will give you some ideas. If you decide to go this route, then depending on what canister filters or canister / HOB combo, this will determine if you need / what size sump / wet dry filter.