Oh MAN!!! Austin is a big cycling city. There are several great shops....Austin TriCyclist, University Cycles are the two I like. BUT, TOTALLY make sure you go to Mellow Johnny's....Lance Armstrong's bike place (I say place, it is a multi-service place).
I don't live in Austin, but we load up the bikes and visit as often as we can, it is a great place for us to go for the weekend for great road and mountain biking.