I would have to say that if you are worried about your FTP Server and the files that it houses I would urge you not to place it on a DMZ that is open to the Public. Consider putting your FTP Server behind some access limits only permitting those that you want to do FTP. FTP is very vulnerable with default settings often cause more holes than you would like to know about.
VPN is a nice solution. You can encrypt your tunnels and then only permit those on VPN to FTP files from the Secured Server. This will limit your holes as well.
You also might want to consider moving away from FTP and looking for another solution. If you files are that sensitive then consider adding multiple protection methods. I am throwing some thoughts on the table below:
-Consider maybe SSH instead of FTP for transmitting your files.
-Look into and consider SSL/HTTPS and using a secured website instead of a FTP Server.
-Encrypt the file itself as well as the session so that if a session was in factcompromised encrypted data would follow and not the actual raw data.
I did a quick look around and you may want to see what these sites have to offer you:
http://www.ssh.com/ - SSH can do FTP ?type? transfers securely called SFTP.
http://www.invisiblesecrets.com/index.html - Encrypt your files.
http://edocs.bea.com/wls/docs70/secmanage/ssl.html - SSL for your Websites.
http://www.openssh.com/ - Open SSH, common for Lunix and UNIX users.
Good Luck!
Jim
