Stephanie,
I have used both the ac & dc type of Radio Shack 4" fans. Granted, they move a lot of air but are quite noisy (compared to similar fans). I decided to switch to a quiter fan, and went with the Coralife Cooling Fan.
I purchased two of these to move roughly the same amount of air as the Radio Shack fan (1), and are much quieter. I chose not to use the IceCap fans because I wanted the fans to utilize use my existing Reef Keeper temperature probe. The fans turn on when my tank temperature reaches 79.9 degrees, & turn off at 78.1 degrees.
The Ice Cap 4" fans are a good choice, they have a fully automated variable speed control with a built in remote heat probe.
I have not done a thermodynamic study on which position is more advantageous for the fans. I tend to agree with Travis. I have had the fans set up to exhaust the air from the canopy. This has resulted in salt coating the fan & mechanism. For the past year, the fans have been blowing air into the canopy with no salt buildup on the fan or mechanism. I have two (2'w x 5"h) openings on the back of my canopy.
Dragon68 has a good idea, but the salt will still get through, if you don't take the time to clean the filter (I had filters on mine).
Ray