Bring 6 quarts water to a boil in a large stockpot. Stir in pasta; partially cover, and return to a boil, stirring frequently. Cook 6 minutes or until pasta is al dente, stirring occasionally. Drain; place in a large bowl. Sprinkle with salt, tossing well; keep warm.
While pasta cooks, bring 1 cup water to a boil in a large Dutch oven. Add clams; cover and cook 6 minutes or until shells open. Remove clams from pan with a slotted spoon, reserving 2 cups of cooking liquid in pan. Add tomato and garlic; reduce heat, and simmer 5 minutes. Stir in fresh parsley and oil; cook 15 seconds. Serve tomato mixture and clams over spaghetti. Garnish with parsley sprigs.