FaceTime only works with 10.6.8 or newer. http://support.apple.com/kb/HT4319
The 10.6 Snow Leopard DVD is back in the Apple online store. You can get it for $19.99. You will need to be running 10.6 to access the App Store. http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
To buy the Snow Leopard 10.6 DVD use the link to the Apple Store above. You would need the App Store to download 10.7 Lion Or 10.8 Mountain Lion.
The Early 2006 model 1,1 Core Duo can only run a maximum of 10.6 Snow Leopard.
The models Late 2006 Core 2 Duos 2,1 through Early 2008 4,1 can only run a maximum of 10.7 Lion.
The Late 2008 model 5,1 Aluminum Unibody through the Mid 2010 White Unibody model 7,1 can run 10.8 Mountain Lion.
To see which model you have go to the Apple in the upper left corner and select About This Mac, then click on More Info. When System Profiler comes up check the Model Identifier and post it back here.