In a pot of boiling water, cook pasta desired doneness; drain, cool, and place in a large bowl. In a skillet, over medium heat, brown meat, stirring to break up, until no pink remains; drain off the fat, then add the taco seasoning mix and 3/4 cup water. Stir; bring to a boil, then reduce heat and simmer, uncovered, for 10 minutes. Add meat mixture, onion, and cheddar cheese to the pasta; toss gently. Add taco sauce and mix gently. Serve on a bed of lettuce, topped with crushed tortilla chips.