Far more in Houston than Austin.
I myself am not Jewish, however I live just blocks away from a Synagogue, and on Saturday and other Jewish holy days, parking and traffic around there gets pretty crowded. Also, many of my friends are Jewish, and they say there is a good Jewish culture so I would assume that, yes, there is a significant Jewish population.
Yes, there is. To find out more, contact the Rabbi closer to you in the Austin area. Here s their contact information; http://www.chabad.org/centers/default_cd��
Do a google search for jewish churches!