Is it possible to purchase the Tarjeta Dorado prior to arriving in Spain either online or at a US travel agency?
Not as far as I know. They want to see your passport to verify your age.
no way at all. You can buy it at the Renfe station in the airport and immediately get discounts on tickets. You can compare TD prices to other fares on the Renfe website, but you can't buy any tickets until you have the TD card number.
I compare the prices. Sometimes the promo fare is worth buying ahead of time. Sometimes the difference doesn't justify getting the very restrictive promo fare, once or twice I bought a more expensive promo fare because there was a high probably that the train I needed would be sold out before I got to Spain.