Personly i woulden't risk using different bios files due to small differences in the GPU's,
Flashing a 512mbs bios to a card with 1gig of ram is asking for problems.
So many things can be different such as the amount of shaders different makes of ram chips different volts and timings.
I really would just try the original bios then install drivers and try changing your background pic.
If that dosen't work then try to at least find a 4830 bios rather than a 4850.
If your card is sapphire i would sugest trying to find the right bios for your card and not one from a different manufacturer.
Have you tryed the sapphire 1gig bios files?