Or you know, just shut up and do it. I mean, for crying out loud, no money down no fuss no muss. This isn't rocket science. click the links and READ.
If you do it in the order listed you can "buy" GTA SA for free and get more coins, to boot.
The only thing worth noting is that the $5 credit only lasts until July 31st and the offer will be pulled at any time because a few games that also work are no longer giving the free $5. You don't have to download the demo or anything. Just "buy" it.
And the Amazon coins are only good for a year, I believe.