What's a recommended setup of accounts on Mac OS X?
All actions taken on a non admin account that would need admin privileges require the user to enter the name of an admin account and the password before continuing.

So if you download software on a standard user account, when you come to install it you will be asked to enter the admin user name and password.

I use the admin account and my girlfriend has a standard user account. She can import music, photos download apps from the app store, she can change system preferences in the settings panel but not alter the status of my admin account, create accounts, etc, etc.

I have not heard of this procedure of using a non admin account as to do anything that will change the system as you are required to enter the password anyway.