No one knows how the 7850 will perform exactly, I personally would wait. I'm assuming the 7850 will have good overclocking similar to the 7970. Maybe wait and see the performace reviews before making a decision.
In general I wouldn't invest in old architecture when there's newer stuff available. For driver reasons mostly (also heat and power) but that's just me, good luck!
If the price/performance of the 7850 is comparable to the 6950 then I will get the 7850 for sure since it is newer architecture. I just want to get the best bang/buck for around $200.
No reviews out yet for it?