Just use your current driver if your just adding another 4850 and enable xfire in Catalyst control centre, thats basically it.
The last hd4850 drivers are 12.6 i believe.
I had a P5Q-E that was similar, with an Ati card with Ati blue screen messages for a few days then would just not boot up
Nothing would get it to boot up, changed for another board and everything was ok again.