Thanks for the reply! I would like to reiterate the key question, though.
PS: I would not worry too much about bottleneck. After all, a good GPU can always be brought forward to the next PC upgrade.
Unfortunately, I won't be upgrading again for a couple of years, by which time both 6850 and 7850 will be ancient and up for replacement anyway. Therefore, I have to make the decision solely in the context of my current system. So, will my current CPU be a significant bottleneck or not?