Originally Posted by davidmcowan
A picture is worth a thousand words:
Gottcha.
Check out this link for an excellent explanation of chain stretch and cog/chainring wear, with photos:
http://www.sheldonbrown.com/chains.html
Ultimately, (as has been suggested above) I would replace the chain first. If you continue to have shifting problems which cannot be resolved with cable and screw adjustments, then I would replace the cogset. As to the chainring, it is a rare feat to actually wear out a big chainring, so I doubt that you would need to do that. But it is theoretically possible that the chainring is worn as well. (Although that does not appear to be the case from your photo).
Bob