Securing SBS 2003 data from internal mis-use or robbery

I have an SBS 2003 server and XP Pro workstations. We have less than 10 users. Our data is extremely valuable and often confidential. I have 2 areas which are currently security holes for data in my network:

1. I assign users particular projects. These users have the ability to work with the data (mostly MS Word docs) regarding the project. Other users must not be allowed access.

Possible solution: NTFS permissions. But this does not prevent users with the permission to access files, copying them and making them available to other persons.

2. I need to prevent that any data from leaving our premises (either via email, copying to floppy, USB stick or even the servers hard disk being removed!). Putting it blunting, I don't want data to be stolen!

Possible solution: Encryption using private/public key rings? Administrative nightmare? Expensive?

Any ideas what measures I can put into practise to allow data just to be used by authorised persons and preventing it from wandering outside of our network.

Thanks for your comments.