I use 2W per gallon regarding heaters and suggest it to others...
I would only suggest using more if you are commonly heating the tank more than 16*F higher than the room temp...
I personally use Visi Therm Stealth Heaters exclusively and have for many years. I'm very happy with them. Although there are other quality brands available (Ebo has a great reputation, though this is not from personal experience).
As DawnMarie suggested, don't 'cheap out' on the heater as a low quality heater can easily cause major problems to your tank...