Well, it's not a restaurant, but there's a nice little place called Taco House near the Hsinchu train station where they serve burritos, tacos, quesadillas, and nachos. All sold from the back of a truck. No place to sit, but there is a 7-11 a few feet away where you can sit inside, eat your meal and watch the language school crowd go by. I had a taco, burrito, and a bowl of nachos the other night for under $200 N.T.
Taco House is located on Dong Men St., #36, extremely close to the Global Village Language Center and the post office (Dongmen Branch). If you arrive by train, walk down Min Dzu Road, then turn right on Dong Men Road.
They're open Monday to Saturday from 4 to 9:30 p.m. Contact them at 0937-995-100 or 0982-871-388.
And if you can read Chinese, here's an article in Chinese about Taco House from a May 2009 issue of Liberty Times: