I am creating an application for Mac using Xcode 4.2, and I wanted to create a page on startup somewhat like the ones in GarageBand and Microsoft Office. The user would be able to select an option (template) and then press ‘Choose’ to start working with that template. The following images depict the startup (option) screens of GarageBand and MSOffice:

Please note that both of these have an interesting way of selecting an option (template) to begin.

I would like to know what kind of UI Element or special class is being used for this. I do realize that a lot of custom artwork would go into this. Please also explain how these items are used like a radio button, not a normal button.
Those are almost certainly done with an NSCollectionView. NSCollectionView can be a little difficult to work with (in my limited experience) so I would start by looking at the docs:
Quick Start for Collection Views
NSCollectionView Class Reference
And then at some tutorials:
NSCollectionView Tutorial for Dummies (Xcode 3.1.3) (A little out of date but still helpful)
Adventures with NSCollectionView