I am not very familiar with the Austrian railroads system, but I took a quick look at their online res system and found out that the full fare oneway Wien-Salzburg is 29 euro in 1st and 18.30 euro in 2nd class. Persons under 26 can get youth discounts, but I guess a special card from the Austrian Railroads is required for it -- maybe it is possible to get it at the station.

I agree with Rudi that there is no need to book a seat in advance for this kind of heavy traffic route. Just buy the ticket at the station when you get to Wien. Also I see no point in paying for first class on Western European trains.