Just flash the 6850 with the 6870 bios. You save yourself money.
You're confusing the 6900 series with the 6800 series. The 6950 can be flashed into a 6970 in some cases, though cards capable of that are getting quite rare due to this trick hurting 6970 sales. The 6850 cannot be flashed into a 6870. The 6870 is about 10% faster than the 6850. For the extra 10 bucks, I'd say it's definitely worthwhile. Even though either card will do quite well at your resolution, the 6870 would last a little longer before becoming obsolete, and will perform better if you do decide to upgrade to a 1080p monitor down the road.