If you get the latest patch 1.1 you have the option of turning motion blur off without having to go to medium settings. There are also other graphics optimisations.
Ajp I don't know what CPU you are running the game on, but I really don't get that bad framerates on high settings with medium shaders on an 'old' overclocked 8800GTS.
I don't think saying CoD4 looks better than Crysis is a fair or correct comment at all. Having played all the way through both I can safely say the Crysis engine is far better, but graphically and for immersion. The CoD engine looks like a updated version of all the other Call of Dutys, but then maybe they've just made it look like that to make it familiar to players of the series.
As I say, grab the latest patch at least. And we're now seeing some community created maps that really show off what the engine is capable of, granted that isn't default game content, but it goes far beyonf what CoD or the Source engine could achieve.