Step 1 : Download 7900GSX.iso and correctly burn it onto a cd. I use http://isorecorder.alexfeinman.com/isorecorder.htm
to burn my iso images. Install program, right click on iso image, and burn to CD. Super simple.
Step 2 : Restart Notebook with CD in drive (make sure your notebook BIOS is set to boot off of a CD first) The notebook will startup in dos and give you 3 options, pick option 2. Hit Enter when the "OK" or "Cancel" window pops up. Now you are left at a dos prompt.
Step 3 : Type nvflash -4 -5 -6 vxxxcxxx.rom to flash your bios. (fill in vxxxcxxx.rom with the rom you would like to flash, such as nvflash -4 -5 -6 v575c775.rom) The notebook will now load the new rom (youll see your numblock-wifi-capslock lights flash randomly) .. then your notebook will reboot with the new video bios installed.
WARNING : Check which bios you have first like everyone else has mentioned. For safe measure, I pulled my notebook apart when I got it to see which card I had. On the video card itself, there is a BIG sticker that says Rev 1, or no sticker at all if you have the older rev 0. ONLY flash the bioses on the .ISO image if you have a rev 0, your notebook will be hosed if you load the wrong image.
If it turns out you have a Rev 1, there are bios images later in the original thread which you can load. These just overclock the 7900gs, as it is impossible to load a GTX bios onto these newer cards.
Hope this helps