so would two 200w heaters be good?
From what I've seen, a heater getting stuck on and cooking the fish is incredibly rare...can't say I've ever heard of it ACTUALLY happening, but it is something I've heard people worry about.
I keep my house at 68 to avoid massive gas bills. One thing though - the wattage of the heater is a function of the difference in ambient air temp and water temp, as well as tank size.
p.s. - excellent discusion
Heater size depends on the temperature difference. Here is a guide I threw together in MS word.