Took the logical route and fitted the 6850 today....
Very pleased , been on it all afternoon/evening.
PSU seems to handle it pretty well no crashes or anything.
i was surprised by the performance gains - I have a stock 2.4ghz q6600 with 4gb ddr2ram.
with the 3870x2 battlefield 3 was getting 25-30 on medium settings.
with the 6850 I can play on ultra at 1080p (deferred anti aliasing off) and it never drops below 40 fps averages about 50 fps
I was surprised because so many forums had said my q6600 would severely bottleneck a 6850