300w would be rather basic. I have the same size tank and a single 300w which works fine right now during summer but I'll be needing another over winter.
Check out the Hydor ETH range - I'm rather impressed with mine - makes sure of a really even spread of heat.
I Have 2 x 200watt heaters in a 300L placed at each back corner of the tank.
At least this way if one was to fail then there would still be a heater in the tank.
They are connected to a seperate external thermostat set slightly lower than the heater/stats in the tank so both come on and go off at the same time.