Prepare pasta according to package directions. Drain and keep warm.
Brown ground beef in a nonstick skillet over medium-high heat until no longer pink. Drain.
Return beef to skillet; stir in taco seasoning mix and 3/4 cup water. Bring to a boil, and cook, stirring occasionally, 7 minutes or until most of the liquid evaporates. Remove beef mixture from heat.
Melt butter in a large saucepan or Dutch oven over medium-low heat; whisk in flour until smooth. Cook, whisking constantly, 2 minutes. Gradually whisk in milk, and cook, whisking constantly, 5 minutes or until thickened. Remove from heat.
Stir in 1 1/2 cups cheese, stirring until melted. Stir in cooked pasta and beef mixture. Sprinkle with remaining 1/2 cup cheese. Serve immediately with desired toppings.