Now i'm wondering, do game demo's reflect how a full installation will run on a computer?
Not necissairly. Demos are often made of betas of the program and their performance is said to be a little worse than the one of the program.
Now, Operation Flashpoint, do you have the option of having the gun in the lower right hand corner of the screen, and the option of looking down the sights when you want to aim accurately?
Of course you have! OFP is IMHO the most realistic war game ever developed. The only chance to hit your enemy is by using the iron sights. And if the enemy hits you - game over.

Additionally,you get the chance to drive tanks and fly helicopters - american ones only 'though.
And the storyline is better than every single Tom Clancy novel out there.
