You will need to edit the config file for the game. This happened to me too. My Documents/My Games/Far Cry 2/GamerProfile.xml, right click and open with either xml editor or wordpad. You'll need to edit this line:
<quality ResolutionX="800" ResolutionY="600"
It will probably say something like 1024 and 972 or some weird number, anyhow, set it to 1920 and 1200 or 1920 and 1080 (or below), save the file, run the game, and you're set. Further graphic changes made in-game as typical.
Edit: that line may appear twice, make sure to change them both. It's a small file, you'll find em.