Nvidia doesn't run AS WELL on AMD computers as an AMD card, but that said the link you posted is for a computer with an AMD processor and AMD card, so no worries.
If you look at reviews and benchmarks, the 6450 is not a particularly powerful card. You might struggle to get really good performance from it playing Arma and GTA. Since the computer appears to come with a 500W PSU, you might be able to switch in a more powerful card like a 7750 or 7770. Those would allow you to play games with a little more consistency.