It depends on what resolution you are running at. For 1920x1080 you can definitely do medium/high for all those games. You won't be able to pull off ultra on Crysis 2 or BF3. Ultra settings on Skyrim does seem to require a lot of CPU horsepower, your card may be able to pull it off, but your CPU may be a bit too slow. Skyrim likes having 2 really fast cores rather than 4 slower cores. If you are running at a lower resolution you will see even better performance with the 6850.
As far as GTA IV goes, it is a really badly ported game that doesn't run all that well even on really good hardware. I have an i5 760 at 2.8GHz with a 6870 and the game will still hit occasional lag spikes when played on high settings at 1080p. Maximum settings won't be possible with a 6850 as GTA IV wants at least 1.5GB of video RAM for max settings, making it one of the only games where it is worthwhile to have a 2GB card.
At your resolution you'll be able to get 35 FPS and up with a 6850 on most of those games on ultra. GTA IV and Skyrim may be CPU bottlenecked at that resolution, and with Crysis 2 you would have to turn a few settings down to get an acceptable framerate. For most titles, the 6850 is actually more than you really need, and your CPU is more likely to hold you back for the most part. If possible, try overclocking your CPU, that will definitely help performance at your resolution.