I doubt the RAM reported in S.P. matters for anything, I think its just the amount of RAM you have physically installed.
Do you get the amount of RAM allocated for the graphics correctly in the S.P.'s Graphic Section? I dont get any Video RAM reported there for my 7100. For me nvkush never goes into graphical mode, although it finds some card with unknown model.
Like I said, 10.5.6 its considered minimum requirement, Apple didnt have any machines with shared memory nvidias til recently so previous versions nvidia kextx probably know nothing about them.