Keep in mind that if you travel back to the US after you have your FBI report, it nullifies the report and you have to do it all over again. So timing is rather important. Also, some people have been told the report is only valid for a few months. That directly contradicts what I was told (which is that it is valid until you travel back to the issuing country) but since it's Argentina, it all depends on the mood/humor of the person at Migraciones the day you go.
It will take a while for the process as well since you have to wait for FBI, then apostille from State, then have it sent here. I had mine mailed to family in the US and then fedexed here.