The 4870's in xfire will probably beat out the one 5850, maybe by around 20%, but that's probably being very generous since most modern games would chew through the 512mb vram buffer on higher settings, then hit the vram dump lag issues. Not to mention you are are soaking up 160w for each card that will most likely barely beat the 5850 with a 150w tdp.
It's hard to compare them since the benchmarks of 4800 series are dx10 and from a different gen; tested using different games. And that was the time when DX10 was just so overrated and people just kept coding for DX9 with little loss to eye candy. The DX10 mode in the few games that implemented the newer API actually dropped gpu performance by 10% when it was suppose to increase it.
Honestly, put them in, test it out and report your findings. Run through a few benchmarks like 3dmark or whatever games you have now.