Probably not much difference in performnace between 7850 and 6000. From the specifications, 6000 looks faster with 3.0/3.1 GHz (depends on model) but 7850 has 2 Mb L3 cache. 7850 may overclock better. I'd go for whichever is cheaper.
(Note I'm running a 6000 and see no compelling reason to go for a 7850).
To be honest you would be better off spending a little more and getting the 955 black edition and an AM3 board. It would future-proof (kind of) your computer and also allow you to get the maximum power out of your video card.
*Overclocking your cpu is ESSENTIAL to unleashing the true power of these new high end video cards. Anything below 3.0 ghz will simply bottleneck the card, the more you overclock your cpu, the higher response you will see from your video card. The 955 Black Edition is perfect for this. Plus it's cheap compared to its sister intel processors.