For video editing and graphics design, I recommend you consider the MacBook Pro. Any of the MacBook Air models you are considering will work, but the MacBook Pro and its ability to address more memory is better suited to these tasks.
Also, the 13" screen would seem a bit small for me for graphics work but this is a subjective opinion. A 15" would be better and if Apple would only make a 17" MBP I'd buy it.
There is a difference between the i5 and i7 but it is not a significant one. The difference in performance is readily apparent only when you compare them side by side.
Today's MacBook Air's are very powerful machines indeed. They can handle graphics design and that kind of stuff, as long as what you do is not really demanding in terms of hardware resources. Mine is running Adobe Photoshop CS3 smoothly. The only problem is that the screen size is small. But you can always use an external display when the need arises. But then you mentioned video. Video is a different matter. I think a MacBook Pro with a discrete graphics card should handle video better.
Just want to also add, that I was able to compared them side by side.
I turned them both on the exact same time for the first time.
1 with a i5 and the other a i7
The i5 was half a second to a second slower to the i7.
I turned thme both on for the first time and did everything at the same time.
The i5 was nearly as fast. I am happy with the i5.