Place potatoes in a large pot of salted boiling water, bring water back to a boil. Cook until potatoes are tender, approximately 15 to 20 minutes. Drain well. Let cool.
When potatoes are cooled, cut them into small chunks. Using an electric blender, whip potatoes with sour cream, cream cheese and garlic salt. Place mixture in a 9x13 inch casserole dish. Sprinkle with Cheddar cheese.