That depends on the type of taco and what they put in it. Is it more like a Taco Bell taco with hard shell or a sof shell? Beef or chicken? Is it more like the traditional "al carbon" taco with soft corn tortilla and grilled steak or chicken topped with cilantro and onion like Del Taco restaurant?
This website: Dietfacts.com has tons of restaurants listed (both Taco Bell and Del Taco are there), and you'll find point value each restaurant item.
I'd stick with chicken if you can and load on the veggies if that's an option. And like you said, have a big salad. Whatever you decide, just enjoy your taco, remember that it should be moderation, not deprivation.
PS I had tacos for lunch too!