If Japan has a total nuclear meltdown, who it will affect. America? What do we do to prepare for exposure?

I am freaking out at everything I am reading in the new right now about Japan likley to have a complete nuclear meltdown--besides the poor Japanese people, who else is this going to directly affect? America? Will we be exposed to more than just radiation? Will a pill even help if people are exposed? The little bit of information I have found all seem to center around a partial meltdown and a radius of exposure...at the end of the day the human race is still learning about nuclear energy and its effects longterm...What can we do to be preventable-gas masks? Something more extreme? I know I am rambeling here but it is one thing to watch a Hollywood movie about what to do or what happens- and a completly different thought process when it is actually happening. I just want to know what "a complete meltdown" means in how it is going to affect everyone else besides Japn.