do you actually intend to spend that much time playing metro though? it was an average game two years ago.
you just don't seem to be very open to advice atm, so if you really want to spend huge amounts of cash on something you most likely will not utilize, that's your choice. But you came here asking opinions and i'm sure most will share mine.
don't get me wrong, I will probably be getting a second 680 on the future. but i tend to enjoy spending large amounts of cash on my pc. the very fact you are here asking the question leads me to believe you are more interested in performance and cost. in which case, a 690 is not going to offer a significant advantage over a 680. especially not a 500 dollar advantage.
Those two games are GPU dependent, but as to what card to get depends on the resolution you plan to run the games at. If your running 1280x720, you could get away with a plain 560Ti and still run those at near maximum, if not maximum quality. On the other hand, if your running eyefinity at 2560*1440 with 3 monitors, you will probably want a GTX 690 to reach playable framerates at max, or near max quality. As far as going with older dual GPU cards, it's not worth at. As of today, a GTX 590 will cost you somewhere between $600 and $800USD, while a GTX 680 costs around $500-$600. While the cost savings is nice, it's boosted by the fact that the GTX 680 with a single GPU not only costs less and takes less power, but is faster in all mentioned games, even when running Eyefinity.