Simple AD Integration using knowledge as authentication key, is for some data to protect not enough. So if you want to raise the bar you add ownership (Smartcard, USB Dongle, e.g.) to the knowledge only authentication system.
So first stone on your way if you use Snow Leopard, you have to be aware that the Directory Utility has been moved from the Utility folder to Core Services. /System/Library/CoreServices/
Then you might want to enable the smart card login feature of Mac OS X using Apple’s nice walk-trough
If can not get you Smart Card based solution running using Apple’s on board tools, you might consider using Centrify’s active directory Implementation:
If you are still unsure what you really need, there is a very good post about alternative authentication methods for Mac OS X from Ryan Faas @peachpit
Of course you might ran in some problems and traps, to prevent you from going wild, there are some ongoing discussions at the Apple Boards: