Agree with what was mentioned in post #4 (two 300 watt heaters). I know someone who only used one 300 watt heater in a 150 gal tank (in a basement)...and had to set it on the max. setting just to maintain 76°F.
Obviously anything more than two 300 watt heaters would work as well...but two 300 watt heaters would be a good place to be. Two 200 watt heaters might be a bare minimum (depending on the temp. you want to maintain)...but two 300 watt would be better.