I was waiting for you to come back on the GRUB option. Realise that you haven't got onboard, but it would still be good for you to use that option and take a look at the xorg.conf file. After building, let it run all the way through to the black screen to ensure it has tried at least once to start X, then reboot and break out during GRUB and choose the recovery mode option. Once you have a shell, take a look at the xorg.conf.
Esp the PCI BusID I mentioned, and run lspci - compare the bus IDs to ensure it is detecting and picking up the card in your "Device" section. Do a dmesg, and comb through the initialisation of your hardware looking for the card and note what it says about it (may be worth doing dmesg | grep nvidia first to get a time stamp, so you know roughly where to look)
Also, in the xorg.conf Device section, ensure the driver says "nvidia", not "nv" or "vesa". Actually, you might want to post the xorg.conf here so we can look at it and check all the other options...
The other thing is to grab the /var/log/Xorg.0.log file as this is the output when X tries to initialise - it should detail where it is failing, if anywhere.
Final question - when you say the screen is blank, is it completely black with nothing on it, or is there a flashing cursor in the top left?
