In general though, a faster card is better for any game. But with DX11, another dimension comes into play: tessellation performance. The AMD cards get pummeled by nVidia when it comes to tessellation performance. If you're playing/going to play BF2/BF3 it's something to think about as well.
Not quite, tessellation performance was improved with HD6000, and the 6870 being the more powerful card (generally) actually matches the 460 is tessellation if not being a hair better; besides, tessellation is just a feature of DX11, not its entirety.