Herndon and South Riding are both good choices. You might also want to consider Centreville. I can't speak to the safety of any area (nor can any Realtor, as it violates Fair Housing laws and is a matter of opinion anyway), but you can look up crime reports for Fairfax County at http://www.fairfaxcounty.gov/police/crime/.
Here's a link for what's available in Centreville: http://matrix.mris.com/Matrix/Public/Email.aspx?ID=29521831171
If you have any questions or if there's anything else I can do, please just let me know. I'm happy to help!