Sorry aout that, i just start rambling sometimes.
Overdriving is when you drive the lights at 2-3 times there normal rated power. Your standard 4' tube is rated for 40watts, if you drive them with a workhorse 7 (electronic ballast rated for 220 watts) you can double the light output. The workhorse 7 is around $55-$60 and will drive two 4' tubes. Get 2 workhorse ballasts and 4 regular output tubes and you can effectively have 440 watts over your tank.
Clear like mud?
Clinton