powerheads are water pumps which you position in your tank to circulate the water and create a turbulance inside the tank. Maxi-Jet powerheads are available in all sizes.
Your aquaclear 500's are moving enough water, but you may not be getting a very good amount of water movement in all parts of the tank. "dead zones" may exsist and are not good.
a couple of small Powerheads placed on oppisite sides of the tank and turned twoard each other will work wonders in keeping the water moving. However, They will not clear the water up. (which is your main problem at this point.)
Check out my gallery to see how I have setup my powerhead. You can see that it is on the oppisite side of the return from my Magnum 350 canister filter. The Powerhead blows down over the rocks and meets the stream of the Magnum in about the middle of the tank. This turbulant area causes the water flow to go every which way, Eventually it all makes it way around, behind, and through a cave to the intake of the Powerhead, Canister Filter, and Skimmer, on the right side of the tank.
The big black box, by the way, is a pre-skimmer, and is highly recommended to improve the eficency of your skimmer. (plus it hides that ugly pump and all of those wires and tubes running from it.)