So all is good with your pc, reformatted and now you are useing the correct drivers for your 4850. Then you are golden, you prob didnt need to reformat your hd, just needed to do a driver sweep. And reinstall the corect driver for your card. But atleast you got it running right!
Prior to this reformat yesterday I had tried sweeping the ati drivers and re-installing them. Same lock ups and freezes.
Then I reformated and it seemed to change everything.
But as Toxic mention, I will most likely be posting later that my pc crashed.