Heat oil in heavy medium saucepan over medium heat. Add onion and three garlic clover and sauté until tender, about 5 minutes. Stir in 1 tablespoon chili powder and cook 1 minute longer. Add tomatoes with their juices and beef broth. Cover and simmer 5 minutes. Puree mixture in blender or processor in batches; return to saucepan.
Season beef with salt and pepper. Mix in remaining garlic clove, 1/2 tablespoon chili powder and oregano. Form beef into 1-inch meatballs. Return soup to gentle simmer. Add meatballs to soup and simmer until meatballs are just cooked through, about 5 minutes. Ladle soup into bowls. Garnish soup with chopped fresh cilantro and serve with sour cream if desired.
