Hi, you could try the IPC 10.5.6 final distro as it gives more options for loading drivers/kexts than most of the others.
Try 9.5.0 kernal,nvdarwin xxx video,intel achx sata driver,patched usb driver,ps2 keyboard(if that is what you have),patch dsdt,applesmbios-27 rev3 macpro or imac. These are just basics to see if you can get past your black screen, you may need to try different video driver. you said in your post that you have nvidia graphics but yur lspci shows ati check your bios to see if the agp/pcie card has been enabled.
Dont load any more than is neccessary for now no audio/ethernet etc, if it works you will have to reinstall to get the rest.
Ensure that the partition you are loading onto is clean and formated to fat 32,
if/when you get to restart press F8 then -f,

Raz

---

PC,(dual boot with ultimate edition 2.4 and IPC 10.5.6 final)
asrock AliveNF6G-GLAN
cpu, AMD Athlonx2 7750 2.7ghz (black edition)
ram, 4Gb DDR2 800mhz
lan, realtek rtl8211b( use nforcelan driver)
sound, on board (ALC662) HD audio (voodoohda.kext)
gpu, Nvidia 8400gs (512mb)
HDD, x2, 250Gb + 320Gb sata
Wireless keyboard (HID device) FK760 (integrated trackball mouse)