The GTX 560 2048MB allowed for pleasant 1920x1080 gaming in Aleins vs Predator and Dirt 2, even with maximum graphics settings, but Metro 2033 reminded that it's still a mid-range card. After all, maximum graphics settings in this game (without PhysX) are too much to bear for some high end cards, let alone midrange. However, that's not to say that gaming at 1080p with the GTX 560 is impossible - you'll just need to decrease detail settings.
You can see that Gainward GTX 560 2048MB at 2560x1600 is the only to score such a frame rate. The two cards with 1GB memory simply could not take what Metro 2033 had in store for them.