Preheat oven to 350°F. Grease a 2-quart baking dish; set aside.
Cook pasta in a large pot of well-salted boiling water according to package directions for al dente. The pasta should still be a little bit firm, since it will continue to cook in the oven. Drain the pasta, then return to the pot.
Stir in the Monterey Jack cheese, cheddar cheese, mayonnaise, and sour cream. Mix until completely combined. Transfer the pasta mixture to the prepared baking dish.
In a small bowl, stir together the breadcrumbs, melted butter, and garlic powder. Sprinkle the buttered breadcrumbs on top of the pasta.
Bake, uncovered, for 15-20 minutes, or until the filling is hot and bubbly and the topping is lightly browned. If the bread crumb topping isn’t as crispy and golden brown as you’d like by the time the filling is hot, just place the dish under the broiler for a minute or two.
Garnish with chopped fresh herbs or a dash of paprika, if desired. Serve immediately.