Do some searches.
But yep basically you will need an over-flow box that hangs on the tank and feeds down to your sump. You can get them at a LFS, online, or build you own. (Search here for DIY Overflow or check about.com.)
Then you will need some type of sump, either a tank or a rubbermaid container. On my 55 gallon I am using a 16 gallon plastic garbage can for the sump because its got a narrow foot print for the amount of water it holds.
You will also need to set-up a return pump from the sump to the tank and can buy or build your own spray bar. You may not want to get rid of all the power heads though. Some of that will depend on how fancy you get with the spray bar returns from the sump.
The last thing to look at on your sump is a way to make sure that if your overflow from the tank to the sump stops flowing you do not flood the main tank. Either a float switch in the main tank, or a lot of people use a baffle type design in the sump so that the pump sits in a small pool of water. As the water flows back into the sump, it feeds back into the pump pool and is pumped back to the tank. If the over-flow stops, the water in the pump pool will pump into the tank and the pump will start to suck air before over-flowing the main tank.
Lots and lots of options. About.com is a pretty good place to get a lot of info as well as these msg boards.
-Scott
