Heres all you need to clean you iMacs screen:
1: Two micro fibre cloths (ensure that they havent been used for cleaning anything else, so they wont have any other cleaning solutions on them).
2: Moisten one of the micro fibre cloths with tap water and wring out as much water as you can (it should be damp not wet).
3: Wipe the screen.
4: Use the dry one to wipe the screen dry.
5: I also have a large camera lens cleaning cloth that I use to wipe off finger marks. I tend to give the screen a wipe with the micro fibre cloths once a month, and in between I wipe it over with the lens cleaning cloth or if I need to wipe finger marks off etc,
I also use the same cleaning method on my iPads screen too.
The cleaning methods I have described will give you a clean and shiny screen. And will do as good a job (if not better) than the expensive cleaning kits, which in my opinion are a waste of money.
Whatever you do, don't use glass cleaning solutions or cleaning solutions that contain alcohol. These can damage the coating that is on the iMac's screen!
Even Apple recommend cleaning the iMac's screen with soft cloths and water, see the below link.
http://support.apple.com/kb/HT3226?viewlocale=en_US&locale=en_US#desktops