Most battery chargers will work on 240v, so you probably don't need a voltage converter -- but you do need a different plug (a round one). Radio Shack will have them, but you can probably also get them at Best Buy.
Budapest is beautiful -- go to the top of Gellert Hill, and shoot across the city on a beautiful day. Or go to the spot just under the Chain Bridge and shoot it at night...gorgeous. The Terror House is not a place for photography...but you should still go at some point to truly understand the history of the city.
I agree with carrying the camera in a bag that doesn't look like a camera bag. I bought a Tenba insert, and took it to the Army / Navy store -- tried various bags until I found one that fit it perfectly. Works great.
Oh...one word of warning. I've been in some rainy places before, but I've never seen it rain as hard as I saw it rain in Budapest. We were there in spring, and got soaked several times. It would be a clear sky and all of a sudden the bottom would fall out. It was as though you were standing under a faucet, it was coming down so hard.
Now, we might have been in town for some odd weather or something -- but a good umbrella, and some protection for your gear might not be the worst ideas.
Oh, and if you get a chance to go out to the Roman ruins and Szentendre, they're both fascinating.