Do your phones have the 850/1900 MHz Bands? You would need those if you want maximum coverage.
The two carriers that uses SIM cards are Cingular and T Mobile. T Mobile has the better deal for pay as you go plans. You can have your own 'unlocked' GSM phone and just purchase the a SIM card with a number and prepaid minutes.
Once again, you have to make sure your phone has the bands I mentioned above to operate in the USA. If you don't have the right phone, then you would need to get another one. In which case you can wait till you get here and get a phone/SIM/minutes from the same carrier.
I currently live in Italy but will be spending
the summer in Atlanta, GA. I have a Sony
Ericsson tri-band phone and a Vodafone number here. I
don't want to purchase a new phone in the States.
How can I use the one I have? It seems that the
pre-paid plans require you to purchase a phone
(and number?) there. What can you suggest? I'll
only be there for two months. Thank you for any
advice you can provide.