Charlie500-
I ride Emma Long (City Park) It has about a 6 mile loop, and if you ride with the right guys who know the area you can pick up around 10 miles in a "loop". Most guys will go out and ride 2-3 loops. It is all VERY tight technical riding. Almost entirely 1st gear. This place is great because it is free and right in the city basically. A group rides it every Wed. night.
Then there is Hidden Falls Adventure Park in Marble Falls. It is a great area with all sorts of riding. $25 for the day. But it has camping, showers, etc. Really nice set-up.
Down by you more there is ZARS which I haven't ridden yet but hear great things about. And also CTOR which I think is up on the North of Austin. Haven't been there either.
Check out TWtex.com and texasoffroad.net.
Foxx4Beaver-
I am running 12/51 and it works really well. All of our riding is really tight technical stuff so the low gearing helps out. I want to eventually put in the WR KDX trans, but that will be a ways off.