How good is this computer for gaming?

Yes you should be able to run most of the games on good settings, but it wouldn't hurt to upgrade the graphics card.
If you're looking to play games like ArmA 2 Operation Arrowhead and ArmA 3. It would be best to have a 4 GHz processor for the maximum performance but that is kind of overkill. There is no need for more than this, but there is room for improvement if you're not on a budget.