For your non-gaming use, the 6850 is way more than enough. And for casual gaming, normal gaming, even interminable gaming, the 6850 is almost always enough. There are some rare cases where the 6870 would be "necessary", but that can be resolved by turning down the detail level in that game.
So, do you think its worth spending $131 more for those rare cases, or would you rather turn down the details a tad in that game? IOW, how much is $131 worth to *you*? Its really your call.