Well, people are talking about different things, I think. The tarjeta dorada is a special card that people over 60 can buy and then use to get discounts off the standard fares. Discounts are 25% for M-F, and 40% on weekends. What I don't know is whether you can use the tarjeta dorada to buy tickets online, or whether you have to buy them in person.
The web fares are something totally different, and many of them are much better deals than the tarjeta dorada gives you. You can get the Web fare and the Estrella fare only online. Until a short while ago, you could only get discounted RENFE tickets on the RENFE website. Just recently, things changed, and several places, like RailEurope and Petrobax (thanks, alipilgrim!) can now offer the discounted web fares as well. In the old days, you paid an arm and a leg to buy tickets through these brokers, but that seems to have radically changed. This is a tremendous change for the better, because using the RENFE website directly is a source of neverending frustration and confusion (see alipilgrim's recent post in this thread).
RENFE tickets can only be bought about 60 days in advance. The cheap web fares sell out very quickly, so it pays to buy them right when they are offered. But if you wait till you're actually in Spain to buy them, the Tarjeta Dorada would be a good thing because it will discount the fare.