On a project I did, I was able to get about sixty samples per period on an 89C52 running at 11.059Mhz. A simple state machine was used to handle zero crossing detection, and after a period's worth of samples were taken a lookup table took care of the square root calculation. By choosing the sample period right (set by a timer interrupt) the number of samples taken in a full cycle gave frequency too. AC current was measured the same way for full AC measurements.