Hi MaryLuke,
I went on this trip last year. In the literature that JY sends out just prior to the trip, it was suggested to take around £150 worth of roubles. I and several others found that this wasn't enough (and I'm not a big spender - I booked all my optional trips before going as well). I had taken along some Euros that I had, and exchanged them for Roubles at a bank.
In the St Petersburg hotel (the Holiday Inn), the desk staff did not exchange money. Instead, they would direct you to a type of ATM which was an exchange machine (in theory - whilst we were there it was constantly out of money). There was also a regular ATM, which was broken. Fortunately there was a bank just a short walk (5 mins) from the hotel which was where we went to exchange our currency. I didn't try to exchange anything in Moscow so can't comment about the situation there.
Obtaining the Roubles in the UK was very straightforward though - I went to the local post office and they took 24 hours to come in.
I used my credit card once in Russia, no problem at all, although I had let the card company know where I would be.
Hope this helps,
RM.