Some explanations for iOS xcasset catalogs required

I am completely new to iOS development. I want to make a small game with the Sprite-Kit framework and I am already stuck at the xcasset catalog.

Say I have a 320x320 pixel image for my iPhone 4s that features a resolution of 960x480 pixels. I put this to "iPhone 2x" in the xcasset catalog, right (cp. following image) ?

Now, do I need for the "iPhone 1x" a 160x160 and for the "iPhone 3x" a 640x640 version of the image? What about the iPad ? What does "iPad 1x" and "iPad 2x" exactly mean in terms of pixels? Is "iPad 1x" also 320x320 and "iPad 2x" 640x640 in my example ?