In a large bowl, combine the ground beef, shredded cheese, ¼ cup sour cream, and 2 tablespoons taco seasoning mix; mix well (the mixture will be soft). Form the mixture into 4 burgers. Grill the burgers, turning once, until done (10 to 12 minutes); set aside. In a small bowl, combine the remaining taco seasoning mix and remaining sour cream; mix well. Place each grilled burger on a bun. Top each burger with a dollop of dressing and garnish as desired. *Optional salsa and guacamole are great additions to the taco burgers.