The HD5670 CF is somewhat equivalent to the HD4870. Its performance varies though. Check out its review here:http://www.guru3d.com/article/radeon-hd-5670-review-test-crossfire/1
It performance (FPS) ranges from something like the HD4870 to the GTX275/HD4890.
4870 is dx10 dude and 5670 holds dx11..............nywayz, crossfired 5670 would be nearly equal to 4850 only if you are running a small monitor.........CF don't help much until and unless you don't want to run a big monitor