Heat oil in heavy large pot over medium-high heat.
Add onions and celery; sauté until onions are golden, about 7 minutes.
Add garlic; stir 1 minute.
Add broth and next 5 ingredients; bring to boil. Reduce heat; cover and simmer until squash is tender, about 15 minutes.
Transfer 3 cups soup to blender; cool slightly, then puree until smooth. Return puree to pot with soup. Simmer until heated through, about 5 minutes. Season with salt and pepper.
Ladle soup into 6 bowls.
Sprinkle each with 1 tablespoon basil and serve.
Per serving: calories, 228; total fat, 4 g; saturated fat, 0.5 g; cholesterol, 0 mg