Why does the breaker trip on my new low-power lighting circuit?
What could be the problem?


It could be that you miswired a switch so that it creates a short-circuit in one position. It could be that the miswiring occured elsewhere (in a junction box, a light fitting, etc)


  Am I overloading the circuit?


No, the total load of 500 Watts is not enough to overload a 15 Amp circuit. At 110 Volts, 500 Watts is only 4.5 Amps.