Rubles -- The ATMs are fine. I've never had a problem. Before you leave, make sure to tell your banks you're going to use your cards in Russia! If you use travelers checks you'll find them hard to exchange except at the hotel, and there you'll get a horrible exchange rate. Purchasing Rubles in advance is probably not necessary as there are ATMs at the Moscow & St.Petersburg airports.
Police -- If you're just in St.Petersburg the chances of being stopped are rather low. I'd carry a copy of my passport with me and leave the original in my hotel. Technically, the police can no longer stop and check you. In practice, they sometimes do. I've never been stopped myself, but I've occasionally seen others get stopped.
Tours -- There are tours available from kiosks in front of
Gostiny Dvor -- right on Nevsky. While I'd trust them to sell you a tour in English if they say that's what you're going to get, don't believe them if they assure you that the bus will be air conditioned. The city is quite walkable and a tour is not necessary. It would be a good idea, however, to arrange a tour of some of the palaces. For Peterhof, I'd suggest getting there via hydrofoil and not bus. That may mean you'll miss the inside, as the lines can be long in the summer and going without a tour means standing in the regular line. The gardens and fountains at Peterhof, however, are WELL worth the trip.