My setup is:
AMD Athlon 64 X2 6400+
2 GB DDR2-800
Asus M2N32-SLI Deluxe
Nvidia GeForce 9600GT
650W Corsair PSU

and I can run Metro 2033 with 1280x1024 DX 9 and almost everything on high (only AA and AF turned off) with around 25-30 FPS.

If my PC (that was more or less up to date 2-4 years ago) can run it in high on this res then your setup should run it on 1920x1080 DX 10/11 and high details without any problems.

Hope this will help, also if you bought the game why don't you simply test it yourself and find the settings you can run with your setup?

That should come close to maxing it, you might have to turn dx11 DOF off for it to be playable (20-40 fps) but even then it'll look fantastic.

Hell, the game looks incredible @ 1280x720 dx11/medium settings.

Why does this game run so much better on nvidia cards?

ATI and nvidia have different architectures, the engine metro uses probably just works better on the nvidia one.

The Fermi architecture is 80% better at tesselation than the previous-generation ATI cards.