Check under color management to see if there is a profile installed for your monitor. Each monitor (type) is different so the video card needs a profile to work with. Check the manufacturer web site for updated profile files.
The spread between light and dark on the screen is non-linear and varies by monitor brand. The gamma setting adjusts the amount of non-linearity applied for a given monitor. Most games (unfortunately not all) also have an internal brightness or gamma setting.
Once you have a profile installed (or even if you don't have one) and set the vertical rate you want you need to adjust brightness, contrast, gamma and any in game settings available. This is an iterative process and will take several passes. You should be able to get desktop and game brightness balanced. Contrast also contributes to "darkness". I leave the video card brighness and contrast settings at factory and adjust the controls on the monitor. My samsung 753DF and 9500 Pro play really well together. Tribes 2, the darkest game I've ever had comes out excellent. Better than with any of the Nvidia cards did.
One problem that could crop up is vertical rate. Faster rates make the monitor look brighter (the phosphors get excited more often). If your desktop is at 85 and you have a game that is stuck at 60Hz the game could appear too dark. You can try dropping back to 75Hz on the desktop and balancing for that vertical freq.
There is a video adjustment tool (lines, color bars, grayscale, etc) that makes the balancing easier but I can't find it on my HD right now. I'll add an update when I stumble across it.