I was there a couple of years ago it was brilliant and I would defintely go back. Accomodation is more expensive at Carnaval time and books out fast so you would need to book well in advance. I managed to find a place for a couple of nights in a hostel just behind Copacabana beach, the hostel was fine, alot nicer than others i've stayed in.
Definitely go to the Sambadromo for an evening- well worth it cos thats what you see on postcards etc. You shuld also try to go up into the different neighbourhoods to experience the more local Carnaval.
Ipanema is the nicer area but Copacabana beach is nice too. Great seafood for $10 at a hotel on the front.
If you have time try and make it to Salvador in northern brazil, the Carnaval there ends a couple of days later than the Rio one. It is a totally different experience. Very African influenced.
The dates for the Carnaval will be somewhere on the internet.
Brilliant, enjoy