Even if you decide to take the train to the city from JFK, you will still have 12 solid hours in Manhattan. Twelve hours is plenty to do a nice walking tour of the city. The subway will cost you $1.50 o/w (or get a $4.00 all-day funpass which you can buy at the Howard Beach station if it has one of the automatic vending machines -- the shuttle bus from the airport is free), a taxi will cost around $40 (inclusive of tip but the toll is extra if you are going to cross the Midtown Tunnel, which is recommended if you are traveling during rush hour) o/w, the Carey Bus to Grand Central and/or the Port Authority Bus Terminal will cost about $13, and then there are other options that will drop you off at the major hotels or other addresses for about $15 (but you should add a tip for the driver). Assuming moderate traffic, the subway ride will take about 1 hour into lower Manhattan, a cab will take about 40-60 minutes and the bus will take 40-60 minutes. If you are getting out of the airport at 8am on a weekday, then I would definitely recommend the subway. In any case, I would recommend the subway in the AM, because there really isn't much else to do in terms of sightseeing so early in the AM and there is no sense in getting into the City when most sights are still closed.
If you are Brazilian and this is your first time in NYC, then you could always visit 46th Street in Manhattan (btw 5th and 6th Avenues). This is the commercial heart of the Brazilian community in Manhattan, at least this is where there are a lot of Brazilian restaurants (Antartica/Brahma guarana and Chopp available) and where folks go to watch the World Cup every four years. It's a short detour if you are going near Times Square or Central Park.
If you are arriving on a weekend, then I would highlight Central Park as a nice place to take a look around (try the area south of 72nd Street). It is also a great place to visit on weekdays, when it is less crowded and you practically have the place to yourself, but on weekends, the park roads are closed off to vehicular traffic and New Yorkers go out and play.
I should add that LIH Prem suggestions are excellent.
[This message has been edited by fallinasleep (edited 04-29-2001).]