Cut 2 peaches into fourths, then each fourth into chunks; place in large bowl. Add chicken and onion. Toss with enough dressing to coat. Cover and chill 15 minutes to 2 hours.
Prepare pasta according to package directions; drain and cool.
Add walnuts and fresh basil to chicken mixture and toss to coat. Add the chicken mixture to cooked and cooled pasta; gently toss and serve! Add more dressing, if desired.