  I use PhotoShop CS0 on 0.00ghz C0D laptop for light work load, and I'm sure the new MBA can handle the CS0 without an issue. You may want to research on any bugs that CS0 may have with OS X Mountain Lion before moving ahead.  P.S: you will get tons of screen real estate with external Display. You may want to calibrate the monitor and change profile under Display when you do any color changes. How important is matching colour with your images and would MBA will deliver to your satisfaction that everyone has different opinion of?  