Hello laukejas,
my english is bad so just a few links for you :
http://www.armedassault.info/index.php?game=0&cat=addons&id=133
The "PROPER Plants" can help too :
http://www.armaholic.com/list.php?c=arma_files_addons_objects&s=title&w=asc&d=45
But the best way to improve the frame rate is diseble the "Shadow detail" in the right colomn of the "Advenced Video Options".
I have a old computer with nforce2 CM, a Athlon XP 3200+ and a AGP graphic card 7800 GS.
I play with the "Normal" default settings, just "Shadow detail" disebled, without addons,
and i have a descent frame rate.
Hope that help.
Have a good day.
PS : Stewy, thanks a lot for yours great "ArmA Photography". :)