Don't listen to all that stuff about it being 110 degrees (43 degrees Celsius) in March. I live in Austin and it honestly does not start getting super hot until the end of April at the earliest. According to Wikipedia, these are the temperatures in Austin in March:
Highest on record: 98 F, 37 C
Average high: 72 F, 22 C
Average low: 51 F, 10 C
Lowest on record: 18 F, -8 C
According to this data, I don't think that you will be suffering heat stroke during SXSW. I would plan accordingly though, so probably jeans, t-shirts, and shorts, along with a few hats. March weather is amazing in Texas, so don't be worried about the temperature. This is coming from somebody who has lived in Texas their whole life. Hope you enjoy SXSW, I know I do!