Everybody should also check this thread :
(the fix is on page 2 but page 1 explains fairly well what is going on...)
There's a 4 lines kernel patch that fixes the allocation of the video memory under the 32bit address space so the nvidia driver doesn't complain about BAR1 stuff.
I managed to patch the 2.6.24-17-rt kernel against that patch and successfully load X with the NVIDIA driver in amazing 1680x1050 resolution!! I still have to find out how to compile the linux-ubuntu-modules to get my soundcard and wifi modules to load.
Will surely post a nice howto pretty soon ...