The 7000 series should run with lower power requirements on average, so I expect the 7850 to require less power than the 6850.
the 6850 has 127 w tdp. amd set their new 7970 with 250w tdp, similar to their previous single gpu flagship 6970. if the new 7850 has 127~ tdp, it will not fit into OP's machine...unless it uses around 60-65 w power.
wait for the 7850 to come out and match the numbers then.
OP's cpu will also cause bottleneck to the 7850 in resolutions ranging from 1280x720 to 1920x1080.
edit: amd has set msrp for 7670 at over $179 at launch. compare that to 6670.http://www.techpowerup.com/156312/AMD-Radeon-HD-7000-Series-Single-GPU-Graphics-Card-Price-Points-Surface.html