This thread was so helpful to me that I decided to write up a bullet proof summary for anyone else having problems. I also have a Quadro FX 1400 and am running Leopard 10.5.1.
01. Install "Apple Graphics Update 1.0" (not strongly needed but may be improve OpenGL speed). Download from Apple direct.
02. Download latest version of Natit, NOT the auto installer, just get the kext from the InsanelyMac forum http://wiki.osx86project.org/wiki/index.php/Natit.
03. If you are not already set up as a root user, open Utilities > Directory Utility from the Applications folder and click the lock icon to make changes. Go to Edit > Enable Root User and reclick the lock icon and close Directory Utility when done.
04. Go to System/Library/Extensions and move the following kexts:
...to the desktop or other location.
05. To proceed to stage 06., right click on the first kext you want to open and select "Open package".
06. Locate the file "info.plist", open it and in the GeForce and Natit kexts search for <key>IOPCIMatch</key> and in the NVdanv40Hal and NVDAResman kexts search for <key>IOPCIPrimaryMatch</key>. Beneath these entries you will find one or more entries of a type <string>0xXXXXXXXX&0x0000ffff</string>.
07. *Add* (not replace) 0x00ce10de&0x0000ffff exactly like that inside the <string> tags so that the above example line becomes <string>0xXXXXXXXX&0x0000ffff 0x00ce10de&0x0000ffff</string>. Note the space seperating each entry (some kexts will already have several entries between the <string></string> tags).
08. Your kexts are modified and now need to be re-added to OSX. First, manually copy them back to /System/Library/Extensions.
09. Next, open up the Terminal application (Utilities > Terminal). Type the following and hit return at the end of each line (don't type the words in brackets):
su (hit return, it will ask for your password, enter it and hit return)
chmod -R 755 Kextname.kext (kextname.kext changes for each of the four files of course)
chown -R root:wheel Kextname.kext (again do this for each of the four files replacing Kextname.kext with the file name)
Now you can quit the Terminal application.
10. Now go up a directory to /System/Library and delete the file called Extensions.mkext. OSX will generate a replacement straight away, don't worry.
My screen went black and I thought it had failed, in fact it loads in really fast after that and its a beautiful thing for a man to behold.
Please post if you find a fault with this and I'll amend it accordingly.