At your current resolution the 6870 will max both the games you listed. At 1080p, you may have to turn some settings down to high in order to maintain acceptable framerates in Shogun II, that game can be graphically demanding.
As far as longevity goes, it will probably last you a while, definitely at least until the next console generation hits, most games are restricted by the aging console hardware and aren't that demanding. There are only a handful of titles out there that really push existing PC hardware. The 6870 won't max out Crysis 2, Battlefield 3, Metro 2033, or Witcher 2 at 1080p, but you'll still be able to pull off high settings. It's a really good card still, especially if you don't have a huge budget to buy one of the new high end cards that have come out in the past few months.