1)use efistudio http://forum.insanelymac.com/index.php?act=attach&type=post&id=30406, work for me as a charm
2)nvkush.kext (google it)

Ihave geforce8500gt 512 -in efistudio use string for geforce8600gt

This thing really saved me

P.S.
You have to delete nvinject.kext!