Actually are you sure I need to flash the 4850? What's the difference between the PC and Mac (now that macs run on Intel)?
Let me explain - I run OS X Leopard on an external drive, on my Dell Inspiron (obviously not mac hardware) and it uses the 8400 GT with no problems with a few extra .kext files.
Could I not just pop the 4870 into the mac and add necessary .kext files!?