I'm not sure you'd need a full 30 minutes to navigate Atocha but I assume you are tlaking about ordering these in advance using Renfe"s TikNet system and you'll have to pick the tickets up somewhere and that will take time, of course.

My overly cautious response would be to book the 12:00...building in for the possibility that the plane could be late; that you may have to wait for a gate or have a "bus gate," that there may be a delay in retrieving luggage/getting through Customs or Immigration, etc.

My opinion alone but base don previous experience and yours could be different.

Karen,

You can also wait to purchase your ticket at the RENFE office in Terminal 1. I just walked up and bought my ticket in November. I found Atocha station to be incredibly confusing, so I don't know where you would pick up your tickets, but once you have your tickets and are at Atocha, you head upstairs and go through security to the AVE waiting area.

It took me 1.5 hours to go from Terminal 4 to Atocha by way of T1 for the train ticket to T2 to catch the Metro and on to Atocha (you do have to switch Metro lines).

you DO realize that if your flight is late your tickets are useless?

i would give myself PLENTY of time.. and if i got in early, try to change it for an earlier train.

I did a similar trip last year--sorry I don't remember the exact time cushion I gave myself but here's the rundown: Getting out of the airport and into a cab was a snap (I had only carry-on). I immediately realized my Spanish is not nearly as good as I thought. In case you do not speak Spanish, the driver will want to know which part of Atocha you are going to, so just tell him where you are going (I assume because the intercity trains are in one building, the long distance trains in another).
I had purchased my tickets in advance from Renfe. When you enter the train station, the ticket pickup area is to the right of the large indoor garden (you'll see). The only confusing part was figureing out which line I needed to be in to get my ticket--I recommend the advance ticket because that line was much quicker. Then we just had to sit in that garden a while and then find our gate. You go through a quick security check.
I'd go with the 12 to be safe, but you shouldn't have any problems.

If the taxi driver asks you which part of Atocha are you going, you just have to say "estacion del AVE" (Ave station), and he will know