I'll be arriving in Vienna at the Westbahnhof. A couple of days later I'll be moving on to Prague by train. Can I purchase my ticket to Prague when I arrive at Westbahnhof or must I go to the station where the train will actually depart to buy tickets? Thanks!

You can purchase your ticket at Westbahnhof (I did this last year)

You can purchase your ticket at any Bahnhof -easy to do at Westbahnhof. Ticket sales are one level down from your Arrival Level - cash or credit card
accepted.

Your Prague train probably leaves from the Sudbahnhof.

Thank you Truffles and Molker!

Speaking of trains and tickets, is it required before you get on the train to have your ticket "validated," or punched at the inside counter? Seems I remember doing this, and not doing it one time, and the conductor wasn't happy with me (dah) I don't want that to happen again - thanks.

No, you only have to validate tickets for the Vienna underground/buses/trams etc, not for the national train journeys.

I don't remember if I validated my ticket on the Austrian railways or not, but I know I did not wait in line or go to some special counter to do so. I don't see how a railway could run if every person had to wait in line to get a ticket punched. If there are validating or punching machines, I would always validate my ticket, why not.

It's a good idea to buy your ticket ahead of time...no anxiety about a long line etc. on the day of departure.
Last week, we went to the Sudbahnhof to buy two tickets from Vienna and Krakow and back. Seemed straightforward, but they were training a new girl and it took her over 20 minutes to get our tickets sorted...we were really glad we hadn't just showed up at the station 15 minutes before the train.