500mA, is a lot. You would be drawing over 4A, at the 3v rail. Driving a transistor efficiently with only a 3v supply, from a NE555, will be hard, and if not done efficiently, your consumption will shoot up... If the 'about 500mA', is on the generous side, then look at the National LM2623A. This will do about 400mA, and will be far more efficient than the discrete solution. This will deliver 300mA, with something around 85% efficiency, with the right inductor/capacitor/diode.