The standard Apple warranty is only applicable to the first year of warranty. If there is coverage with Apple Care then the warranty is extended to 3 years from the date of purchase. Since you have a late 2009 all of this is probably moot as more than three years have passed. By upgrading the RAM yourself you have not voided an Apple warranty.
This link is to an Apple site to enter the serial number of the MacBook and confirm the status of the Warranty.
Check Your Service and Support Coverage
Review your Apple warranty status and eligibility for support and extended coverage.