If you have a home in an area with good, quality schools you're in good shape!
The Northgate schools are already desirable and high caliber, and if switched into Walnut Creek and Acalanes school districts I'm not sure how much more value that would add to your home. Northgate homes already command higher prices than anywhere else in the Mt Diablo school district.
Look at Dublin v. Pleasanton schools, Antioch v. Brentwood schools or parts of Castro Valley that fall into the Hayward school district. There are examples of it all over the Bay Area. Even within a disctrict - look at the premium that homes in the "Mission" district in Fremont get over the neighboring schools (Irvington) in the FUSD.
So yes, it has an impact. Find yourself a knowledgable Walnut Creek agent who can show you the numbers.