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Ok, so I just found a new heater... it's called the Fluval "E" Electronic Heater and can be seen by clicking this link. Anyway, I'm curious to see what you guys think. I only bring it up because while it looks and sounds nice and even has the digital display on the front one would beg the question of how accurate it is in displaying the overall tank tamp. By that I mean say you have a 4 foot long tank or longer. Well, if the heaters at one end hows the "system" going to know if the tank temps even all around or if it's just reading the immediate temp around the unit.

Edit: I guess you could solve that problem by putting your pumps outflow near the heater since the intake would most likely be at the opposite end. This way the water that would be hitting the heating unit would be water fro mthe other end of the tank which naturally would be cooler than the water surrounding the unit itself on a large tank (4 foot or longer).

Edit: Here's a bit more info I found on it that makes it seem even that much more kool.

These submersible, electronic heaters are equipped with an advanced digital microprocessor monitoring system with dual temperature sensors and utilize VueTech technology that provides an LCD display that changes color if the aquarium water changes +/- 2ºF from the preset temperature. If the temperature changes by more than 5.5ºF the colored screen will flash indicating a potential risk. The LCD is Green when the temperature is safe, Blue if it is too low, and Red if it is too high. Each heater features a heavy-duty, plastic fish guard to protect fish and invertebrates from coming in direct contact with the heater core or glass sleeve. Heaters also come with a secure slim profile mounting bracket with 3 suction cups. The heater simply clips onto the bracket for easy adjustment or removal.

Regardless of the heater you choose, I'd buy at least two and have one at each end. With such a long tank you're going to have quite a temperature gradient otherwise.

What filter do you use? If you have a Rena XP canister then I'd recommend looking at the Rena Smart Heater. It takes the place of the intake tube so all the water drawn into the filter has to pass through the heater first which results in more even heating. http://www.marinedepot.com/ps_ViewItem~category~Aquarium_Pharmaceuticals_RENA_SmartHeater_Saltwater_Aquarium_Supplies_Heaters_Inline___Specialty~vendor~Aquarium_Pharmaceuticals~SearchStr~rena+smart~action~view~idProduct~AP7151~idCategory~FIHTIH.html

There is also the Hydor External heater which can be used with just about any canister filter. It gets spliced in line on the output hose of a canister filter, but on the outside of the tank. http://www.marinedepot.com/ps_AquariumPage~PageAlias~heaters_hydor_eth_inline_in-line.html

I've heard a few opinions on them. Everyone I have read using them likes them a lot.