The main GSM networks are Cingular and T-mobile, with some virtual operators. Or there are also CDMA phones on Sprint, Verizon etc.
Take an unlocked tri- or quadband phone, as they use different frequency bands to us 850 and 1900 MHz, or buy a $20 or $30 one when you get there.
SIMs are widely available and call rates are in the range 10 to 17 cents - charged for both incoming and outgoing calls.
compare on
http://www.prepaidgsm.net/en/usa.html