You might need to get some Hungarian Forints, e.g. to pay in really small shops like a bakery or public toilet. But all restaurants, shopping malls and taxi companies accepts cards (Mastercard and Visa). If they are not accept card, it is better to forget them. Note that American Express is hardly used in Hungary and cheques are not accepted at all. If you forget to change money at home, you can still do it at the airport at any terminal at Interchange office or you can ask the receptionist of your hotel for a money change office nearby to the hotel (there are many in the city).
Currency exchange at the airport: There are several places at the different levels of the terminals where currency can be exchanged. For details, please see http://airportbanking.com/budapest-ferenc-liszt-airport-bud/
More general information about the currency can be found here: http://visitbudapest.travel/budapest-info/currency/