6850 isn't the right upgrade for you.
You will need to spend some more money.
As for the bottleneck no it won't be bottlenecked.
As long as you are playing at a high resolution you should be fine.
A coworker has a 6950 2gb with a 8400 after moving from a 88gts 512
he says it is like night and day.
6850 will be on par with your current cards, you will however gain dx11
maybe look at a 560ti or 6950 1gb