If you've installed the NVidia drivers from their website
, then you really shouldn't use config tools like Sax, Sax2, Mandrake Control Center, etc, you should configure it manually. Honestly, it's not too hard. Make a backup of your currently working /etc/X11/XF86Config-4 file, then
edit it and change the relevant sections at the bottom. If you get into problems, post here.