Assuming, This is only your nvidia problems here are some possible ways.

Try booting into safemode and then
1. EFI string with Custom. (Dont select the readymade card)
2. Any of the patched nvidia Kexts (Remove NV* abd GE*), Freshly install the patched kexts. You can get here in downloads section or search for 9f23
3. If yours is 512MB..it might need ROM Modding. search for "Nvinject"+ "512"


If its not a problem with Nvidia..then you can try
1. Use different kernel
2. Use different AppleSmbios kext

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Ideneb 1.4..Voodoo kernel with 10.5.5 seabelt fix.
AMD 64 Athlon 3000+ 3.5GB RAM
Seagate 500GB SATA...Seagate 160GB SATA.....Seagate 40 GB non SATA....320GB IO-DATA USB external...Logitec 80GB External USB
XFX nvidia 8600 GT 512MB..QE/CI enabled with 9f23
ADI AD1986A(Modified version of 1986 AppleHDA) all jack working, but without any volume controls
Nvidia Ethernet ..nforceLAN kext
Everything else works!!