Your first job is to Google those games and "system requirements". That'll take you to pages like this which will tell you the minimum and recommended hardware. Generally, I'm guessing you'll find that it's the relatively cheap graphics card in the configuration you're talking about that lets you down. In the case of Arma II, for example, your proposed system meets the minimum spec for the game (which would give you a barely adequate gameplay experience) but falls well short of the graphics card noted as 'recommended'. The processor and memory in the system you have in mind are good; I'd have questions about the power supply (it's adequate wattage, but doesn't specify the manufacturer and it's an area you don't want to skimp); but the graphics card isn't a very good one.
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