Don´t forget to pass the signal you are trying to measure through a suitable low-pass filter before your ADC converter sample it. Remember that frequencies higher than your half salmple rate must be eliminated to avoi aliasing. If you plan to measure the power line (mains) voltage, think about galvanic isolation for safe reasons.
I have used a 32 samples per period RMS voltage meter using an 89C51 with fine results.
And if you add current measuring you can get the active power and power factor too.