Place 2 cups potatoes in a saucepan; add water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender. Remove from the heat (do not drain). Mash potatoes; stir in butter.
In a 1-1/2-qt. slow cooker, combine the ham, celery, onion, garlic powder, paprika, pepper and remaining cubed potatoes. Stir in mashed potatoes; top with cheese. Cover and cook on low for 5-6 hours or until vegetables are tender. Stir in the sour cream until blended. Thin soup with milk if desired.