Of the two cards, the 6850 should perform a bit better than the 550ti in general, but they are more alike than different. But for BF3 that may or may not be the case.
Check out some specific BF3 benchmarks.
But, I don't think either card is a big enough jump over your sli 8800gt cards to make you really happy with the upgrade.
The recommended requirements for BF3 is a GTX560 or 6950 class card:http://www.enterbf3.com/information/538-minimum-requirements/
For highest settings, I suspect you may need more.
Your pc meets the minimum requirements, so you may want to wait and see how you do before deciding on what upgrade you may or may not need.