Originally Posted by SQFreak

I used that too, but they wouldn't sell me the SIM at Domodedovo or at the store until my visa was registered. Once my visa was registered, and after some rather incredulous due to the fact that my passport was issued at the New Orleans Passport Agency (it was a few months after Katrina), the MegaFon store at Okhotny Ryad sold me a SIM.
The top-up time is dependent upon how much you put on it at a time, if I recall.