Where you will stay in USA ?? As you know USA is a big country.Some area got no mobile coverage.
You can bring your unlocked phone and use in USA with AT&T. Basic need GSM 850/1900.
Most Smart phones brought in Asia will work with 3G or H++ with data.You only need to play around with the APN setting.
Don't waste money buying sim cards off ebay ,they are free from any Corp. store of AT&T.