What is the cheapest way to phone the U.S. from Spain? Do I buy a phone card, rent a cell phone? Any suggestions will be appreciated.

I usually buy a Telefonica Phone Card (the one with the I/C chip on the card). I've called the U.S. from Spain with one of those for as little as 1,30 Euro. Of course it depends on how long you talk. They are available at Tabacs. I also bought one at the newsstand in Sevilla's Santa Justa station. You can buy them in several denominations - I think the last one I bought was for 6-Euro and it gave you 1-Euro more of talk time.

Calls made with cell phones in Spain are very expensive. They are only worth to call other cell phones or receive calls (incoming calls are free in Spain).

Thanks for the info guys. Eddie what is an I/C chip ?

It's an Integrated Circuit chip that retains info on how many Euro's credit you have remaining on the card. It's on the topside of the Telefonica phone card toward the front: a little gold thingy about 1/4 inch square.

Just ask for a Tarjeta Telefonica. It comes in a little plastic wrapper and you can see the I/C chip thru the plastic.
P.S.
When you call to a cell phone in Spain it costs as much as it costs to call to the U.S.