Sounds familiar to me.
On my sony Vaio i´ve got nearly the same problem. i.e. my fx5600go works fine with the standard "nv" drivers, but a black screen ( sometimes with some nice Stars of fragmented text) locks my notebook completely. Just about no time the xserver is started under the newest nvidia drivers in order to activate my open gl support, nothing goes.
Thank to that i got a quite fundamental introduction to the x window system, what covers my lack of knowledge in other parts of linux, what maybe also linked to the fact im using it for aproximatly three weeks now. Its a suse 9.0 installation, and if anyone got a clue what -else- i could do, i can offer to post my XF86Config and the XF*.0.log if needed.
My next guess is the AGP configuration, but that is only a guess from a newb.
If somebody knows a link to some fundamental infos to the options given in the XF86Config I would be very pleased either.