Just remember that you need a letter from your bank manager to certify the amounts held when you apply for the extension... These letters are only valid for 2 or3 days... so the branch needs to be close to your Immigration office...
You can bring in 500,000 Bt in cash without having to declare it... Open a foreign currency account in Daung (Cambodia) in THB... transfer the money there and walk it across the border... don't tell your GF where you hid it...