To be unique, i will try and answer your question.
Most every restaurant will take credit cards, but never hurts to ask before you order.
As for how much to bring in Forints? Who knows. What do you plan on doing?
A taxi will cost you about 8,000 from the airport to most tourist zones in town.
A 72 hour TravelCard will cost 4,100 forints and will take care of all your transportation needs (sorry no 48 hour card)
Food will cost .... well, depends on what kind of food. This place is sort of located between the tourist zone and the real zone so figure 10% more as you get closer to the river and 10% less a few blocks in the other direction and off the major roads: http://lugas-etterem.com/etlap/
If you do end up with extra forints I can direct you to a mail box not far from the opera where you can leave them........