The resolution of the 10-bIt ADC is 1023 steps. That's about 5mV with the 5V reference.
And, use type float like the [u]Analog Read Voltage Example[/u] if you don't want to loose resolution.
int conversionFactor = 3.3/1023;
That's not correct with the default 5V reference (and you shouldn't be using in integer).
You can change a parameter in [u]Serial.print()[/u] to determine the precision of the printed result. (But, don't be fooled into thinking you can get more than 10-bit resolution from the ADC.)