TGZNOL you are experiencing something normal, ArmA2 demo runs well due to the fact that the maps are smaller, you don't have the wildlife module working, the civilian module and alot of other things that exists in the final game.

There are also reports, of optimization problems with the 2xx series of the nvidia cards. You can read it all here: http://forums.bistudio.com/showthread.php?t=76908

The first thing I suggest is doing the following steps:

1 - Go to the nvidia control panel -> manage 3d settings -> Program settings (chose arma2.exe) and in the bottom put vertical sync in force off. This will boost your FPS for sure.

2 - There are also reports that if you set the maximum prerendered frames in the same panel then 1, will boost your fps.

3 - In the same panel, make sure that anisotropic and antialiasing are set to application controlled.

4 - Create a shortcut in your desktop for arma2.exe, open the properties with the right mouse button and add the following commands to your target line:

-cpuCount=3 (this will force the game to use the 3 cores, if you had a quad core it would be 4)

-maxmem=2047 (this will force the game to use 2 Gb of ram, when in default it uses allot less.)

Target line should look like this: "C:\Games\ArmA 2\arma2.exe" -cpuCount=3 -maxmem=2047 -nosplash

5 - Try to play with the nvidia drivers, revert to 182.5, or try the latest ones, to see what fits you best.

6 - Pray, that the game doesn't take long, as ArmA(1) did, to get well optimized...

I hope that this helps you out.