Hi there!

Finally I solved it for myself!

What Did I do?

add devid to NVDANV40Hal.kext and NVDAResman.kext 0x029110de

Applied Firmware patch for graphicscart: Source http://nvinject.free.fr/ROMs/G71Boar...SI_7900GTO.zip

Used nvflash for windows -> commandline tool http://nvinject.free.fr/files/nvflash_for_windows.zip

Be sure to delete nvinject.kext from your extensions.
Add hardware efi string with efistudio/OSx86 tools. Mine is now detected as an 7900 gtx with 512 MB. Works fine for me.

1. Boot into windows
2. unzip nvflash to c:/nvflash
3. unzip G71Board-p348h24_MSI_7900GTO.zip to c:/nvflash (filename MSI_7900GTO.rom)
4. check nvflash detects your card and on card rom chip with :
nvflash --check
(this will list nvidia display adapters, and supported on card rom chip)
5. compare with the ROM you dumped earlier to make sure everything is fine :
nvflash --compare BACKUP.ROM
if you get any mismatch other than filesize mismatch, reboot with your untouched card and try again.
6. once you're 100% sure nvflash works fine on your card, flash your card with the fixed 512Mb ROM
nvflash --eraseeeprom
will blank your current nvidia card ROM
nvflash --index=0 -4 -5 -6 -j MSI_7900GTO.rom
--index=0 switch can be --index=1 if you have another NVIDIA card as primary display device (PCI or onboard)
you should not get error message at this point. if you get error message, try again without rebooting.
if errors keep on showing, reflash with your original BACKUP.ROM and compare to make sure you did not kill your card
7. if flashing with MSI_7900GTO.rom went fine, do a last check with :
nvflash --compare MSI_7900GTO.rom
just to make sure...
8. we now suppose you did not get any specific error while flashing your card, so just cross your fingers and reboot.
Darwin Bootloader should now detect 512Mb VRAM correctly.