After spending the last month or so doing some serious testing with different OS versions, it really depends on what your hardware configuration is and also the situation you are intending to use Catalyst for.
10.6.4 with the graphics update works well with Nvidia(GTX285 and GT120) cards once you have all the firmware and drivers upto date on late 2009 early 2010 Mac Pros.
Each OS version has pluses and minuses, and apple seem to be dropping the ball on how stuff gets fixed or broken.
Oh and AIC is not a standard codec that is loaded on new machines... you either need to have a application that install it or do what Nev said.
Phil
Philip G Haynes
Live Visual Design and Direction