You need to count up all the power used on that circuit. Figure out which breaker that circuit is on, turn it off and find all the fixtures that no longer work. Add the wattage together, divide by 120, the result is the number of amps used by that circuit. Your 600 watts of light equals 5 amps (600/120=5), so see if adding 5 amps to the circuit will overload the breaker (probably a 15 or 20 amp breaker). I try not to use more than 80% of the total capacity, but that depends on if you're likely to use all the lights/appliances/whatever at one time.
As far as safety is concerned, as long as you have the right sized breaker on the circuit for the wire, you won't start any fires. The worst that might happen is you will be blowing the breaker. The more often you do that, the weaker the breaker will get, and the more often it will blow. On that note, if you think the circuit is close to but not overloaded, and the breaker blows, you might have to replace the breaker. They can get weak over time, which is one reason I don't like to go over 80% of capacity.
