My setup is as such:
Core i5 2500K OCed to 4.5Ghz
Asus P8P67 Pro
GSkill 1600mhz DDR3 running at 1866Mhz - 4GB
EVGA GTX 570
I ran Metro yesterday on my GTX 570 OCed from stock clocks 732Mhz Core to 875Mhz with the shaders running at 1750Mhz and RAM pushed slightly to 2050Mhz - so the card performance should be tied with the GTX 580 at those clocks.
I ran it at 1920x1080 and was getting around 47-53FPS with everything Maxed except for AA which was at AAA instead of 4XMSAA. Switching my resolution down to 1680x1050 i was getting about 55 fps on average peaking about 60 and dips to 45 during combat scenes, looking at your watch, or other animations. Mind you, nVidia cards run with Advanced Physx on with DX11 settings, if I had switched it off, I probably would have gotten higher frames.
I'd say getting a minimum of 40fps at Metro is sufficient, but like any other performance gamer, a constant minimum of 60fps to maximize monitor usage and fluidity of game performance is heavily desired. At 1920x1200 and higher resolutions, the GPU usually becomes the bottleneck. Metro is currently the most demanding game, so to be able to run it at a higher end of the resolution spectrum available today (1920x1200 is more taxing than 1920x1080 and you will see at least a 10% performance dip moving from 1080p to 1200p.
If your end goal is clearly defined as 40fps with max settings (including the heavy contested requirement of 4x MSAA) you need more power and a 6950 will not cut it even if unlocked to 6970 shader specs and clocks and even OCed. Average of 40fps is a goal. But dips will be inevitable to the 20s in more complex scenes, which is why it is only the usual case that people target 60fps as the goal, because anything below 40fps is visibly noticeable that performance isn't fluid.
Putting 2 6950s unlocked and OCed in CrossfireX would effective double your framerate, as AMD 6000 series cards scale incredibly well in Metro, and in general have improved greatly over the Cypress 5000 series. At 1920x1200.
Anandtech - A Look At Triple-GPU Performance And Multi-GPU Scaling, Part 1 This link will show you performance of scaling with 2 cards and 3 cards of the biggest Giants from the Red and Green teams. At 1920x1200 resolution, two 6970s scale acceptably from 39fps with 1 card and going up to 66fps with 2 x 6970. The graphs does display diminishing returns moving up to the third card, but I do know of other users getting 100% scaling in Metro 2033 with GTX 580s Tri-Sli and Quad-Sli all the way up at crazy surround resolutions like 4800x2560 which is effectively 12 Megapixels. This is however done with a i7-2600K OCed to beyond 5Ghz or a Gulftown 980X OCed to a minimum of 4.7Ghz (reduced or eliminating any CPU bottleneck, that this review might have faced when using an Intel Core i7-920 @ 3.33GHz only - - which would definitely impede tri-sli or tri-fire configs at 2560x1600 (4 megapixels).
But coming back down to earth, a single card configuration at this point in time that would ensure a minimum of 40fps in Metro 2033 at 1920x1200 with everything Maxed out (including the 4xMSAA which you can afford to sacrifice) currently does not exist. (I do not consider the GTX 590 or AMD 6990 to be classified as 'single cards' even though they physically are - they are dual GPUs and in most cases their price point is higher than buying 2 cards for crossfire/sli.
A single card setup will not require you a platform upgrade, and as you have seen - your game performance at 1680x1050 is satisfactory, and will definitely see great results with a CPU overclock to beyond 3.8Ghz on that dual core. Metro has been tested in a Tomshardware article on CPU bottlenecks, and it was concluded that Metro really benefits from a Dual core, and not so much a tri or quad core cpu. The actual GHZ speed of the processor is more effective and translates directly into performance instead of the number of cores.
If you need to pursue that goal and achieve 40fps at 1920x1200 resolution with 4X MSAA, you would definitely need a dual card setup based on cards currently available. A single card will be intensely strained and maintaining a minimum of 40FPS with the kill 4xMSAA is simply not possible with today’s offering.
Think carefully about your needs before assessing and planning an upgrade. In most cases, bragging rights do little when actual gameplay experience is all you crave for.