5 watts per gallon is kind of a safe rule of thumb. Obviously if the tank is in your basement, which is typically a lot cooler than say, a bedroom or kitchen, you will need more heating. I was contemplating a 150 watt for my new 44 gallon, because my bedroom is fairly warm (typically at 74*f) so the heaters won't have to work so hard. I bumped myself up to 200 watts when i realized i sometimes have the habit of leaving my window open a crack at winter (like last night) and now my room is freezing, first thing in the morning. (sure makes it hard to get out of bed!)
I also saw a guideline that based watts-needed by difference between room and water temperature. I'll try and find it again.