I guess you should check this thread -
http://www.prepaidgsm.net/forum/index.php?showtopic=513 , it's long enough so there you may find all the answers that you need. Although it's about a year passed since the time the thread got started, there's nothing actually changed. I still recommend you to pick the Единый (Ediny) tariff from MegaFon in Moscow, and a local St.Petersburg SIM from any operator + a calling card to call the States. Will you need to call local Russian numbers often? Well, if you need some help with Russian (constructing the phrases that you'd use during a SIM purchase), I'd gladly help.