Measure AC Voltage using this circuit & Microcontroller ADC
At the node A/D you have 2.5Vdc when IN is 0V. When the AC waveform is at its maximum peak value of 500V you have approsximately: 2.5V + 500V * 500/10^6 = 2.75V and when it's at its minimum 2.5V - 500*500/10^6 =  2.25V. So I would try to set the ADC supply voltage at 2.75V and its reference to 2.25V with respect to ground in order to make the most of its resolution. If you need some margin you can put 3V to the ADC supply pin. 
Sampling frequency should be at least 500 samples/s in order to reconstruct a useful signal. The maximum sampling frequency is imposed by the time the uC takes to make calculations using the actual sample. You should measure it in order to maximize sampling frequency which is always useful. You should also put an antialiasing filter at the input based on the sampling frequency you are using. Its time constant should be greater than double the sampling time in order to meet the Nyquist criteria. 
I hope this was the answer you were expecting.