Metro 2033
Set in the shattered subway of a post-apocalyptic Moscow, Metro 2033 is a story of intensive underground survival where the fate of mankind rests in your hands. And if you happen to enjoy your on-screen visuals, it's one of the best-looking games we've seen in recent years.
Metro 2033 does a good job of spanking AMD's new GPUs; you'll need an HD 5770 or GTX 550 Ti to play at these medium-quality settings.
The HD 6670's significant lead over the HD 5670 is difficult to explain; perhaps the extra shading power really counts here.