As Norma said. Tell me where you are staying and maybe I can tell you where the nearest ATM is located. Most if not all of the larger establishments will take credit cards. You know the rule: if asked you want YOUR bank to convert the funds, not the proprietor's bank. Makes a big difference in the exchange rate. If not asked you can be pretty safe in assuming that YOUR bank is the default. The Christmas market in Budapest opens mid-November so you should get to see that too. I generally spend cash because I have it, but I am pretty certain that most of the major vendors at the market will take cards as well.
A few pics from some past Christmas' http://1drv.ms/1VREs5q
Christmas is a good time of the year to be in Budapest. The theaters are alive, the restaurants cozy and the lights spectacular. It can be approaching 10F or sweater weather. There can be a foot of snow or clear and dry. But its always good.