Hd 6870 is much better hd 6850?

you will get good performance with both in that game as long as you are running a quad. The 6870 is however, undeniably faster than the 6850, and you should look into maybe games that you will play in the future that might need the extra juice
 


How much is much?
The 6870 is better, but not a lot better.
If you had a 6850, you would not bother to change it out for a 6870.

If you have the budget, get the best card you feel comfortable buying.
a 6950 is better than either, and a 6970 is better yet...... You get the idea.
 

Every step is around 10-15% faster before overclocking