You should note that in order to make HD video calls you will need to have a built-in FaceTime HD camera and 1-Mbps upstream and downstream Internet connection. For normal video calls you will need to have a built-in FaceTime camera and a 128-Kbps upstream and downstream Internet connection.
FaceTime is compatible with Mac OS X version 10.6.6 or later. It is 16.8 MB of size. FaceTime is available through the Mac App Store in the Social Networking category. It costs $0.99 USD. You can download and get more information about FaceTime from here [iTunes Link]