If the water temp you need in your tank isn't more than 10 degrees (F) (or about 6 C) above the coldest air temp of the room the tank is in (how cold does your house get in winter?), then 3 watts/gallon should be sufficient - call it 450 watts total. Any more than that and you'll want to go for 5 watts/gallon - about 700 watts.
So far as multiple heaters are concerned, I agree with the poster above who say multiple heating units. If you have large seasonal variations in the air temp in your house, you can purchase a 250W and a 300W, and use the 150, 250 and 300 in winter (for your 700W winter need) and then pull the 250W or 300W when summer rolls around (when you only need about 400)
At regular household voltage (call it 120V) keep in mind that at 450 watts, you're going to be pulling about 4 amps on the circuit and about 6 amps at 700 watts. Most household circuit breakers are rated between 10 and 20 amps per circuit, so you might want to check your circuit breaker box too (qualified electrician if you're not sure - safety first, last, and everywhere in between). You don't want to set everything up in that big beautiful tank only to find you can't turn on the TV...
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eHow: How to choose a heater for an aquarium)
