If you have a GTX2xx card and can't boot, you need to delete the following kexts (or move them somewhere else) in single-user mode.
Geforce.kext
NVDANV40Hal.kext
NVDANV50Hal.kext
NVDAResman.kext
Also remove any injectors you may have.
Then you should be able to boot into safe mode at least. After that update to 10.5.7, then remove the drivers again to boot. Then you can install the drivers. You might want to install Chameleon EFI 2.0 as well.
Instead of the injector you may try installing an EFI string into com.apple.boot.plist. See instructions
here. They are for a single GTX260 896MB so for other cards you may have to change the VRAM value.
In any case you shouldn't need to use VGA cables. If after using an EFI string you get no display it means there's something wrong with it. I had to try a few ways before it worked.
At the moment my Hackintosh 10.5.7 is working perfectly except System Profiler still shows "there was an error" in the Hardware tab and I can't get sleep to work properly (no display after wake up).