1. Try to first remove all nvidia and geforce kext's from your machines System/Library/Extensions folder (restart). 2. Then use EFI studio to write your 7600 512mb to com.apple.boot.plist. 3. Finally, install the 9f23installer.pkg for the correct 10.5.6 kext's for your nvidia card (restart). You should be good to go.

This is what worked for my 8600 GT 256 card in iPC 10.5.6

If all else fails, reinstall iPC without selecting anything from "customize" for your video drivers (no kexts, nothing). Then continue from step 2. above