Do you want something that follows the envelope by itself, doesn't require any commands from the microprocessor, but forces you to compromise between error and speed, or do you want something that does require some interaction with the microprocessor but will acquire a true maximum and minimum within one cycle of the AC?
For the first you don't even need a sample & hold, per se, you just need an amplifier followed by a plain old rectifier circuit:
A = 500 Peak |\\ .-->|--------------o----------- Signal In || | \\ | | o---------||-----| >-----o | || | / | | Trough |/ '--|