I moded Sapphire HD4850 bios:
1. Upper limit GPU set to 950MHz
2. moded powerplay: in 3D set 1.158=1.21V (monitoring) - it's max value for Vgpu in bios. If value more than 1.158 - overprotection kick voltage to 1.1V in monitoring

3. fix a bios checksum.
original bios
get it
here or
here
My bios work great with Catalist 8.7 in Windows XP and Vista x64, and don't work with catalist 8.8beta
Enjoy
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