ArmA doesn't support dual core , don't know if it can be patched later to do
this.(or maybe it has already?)
http://community.bistudio.com/wiki/ArmA:_System_Requirements
Your bushes slowdown apparently will be most affected by your shadow details
setting.
I've seen some posts suggest set refresh rate in game to 75 (if it's set
higher than that), dunno why?
Apparently in the Catalyst AI settings, set it to aggressive and it uses
alternate frame rendering, dunno if thats better or not.
I'd see how it affects any other games you have, maybe it's just
ArmA....rather than your set up.
--
RobertJM