The yesss.at website is particularly easy to follow if you use the beta version of Google's Chrome browser with language translation. The 3 (drei.at) website is a bit harder to translate because a lot of text is in graphics.
With yesss.at you can get a SIM with 512MB for EUR10
here. You can buy these at Hofer stores in Austria or order them online providing they're shipped to an address in Austria. I intend to do the latter, having them ship to the hotel where we'll be staying.
Here's what you need to configure your phone to work with yesss.