ok this is what u need, u need to get the Nvidia driver from Nvidia, and also the kernel update from nvidia, for your system e.g there are several diffenet one, if u have an athlon smp system MAKE SURE U GET ATLHON SMP and not i686 as it will not work, so for example, i got the nvidia drive for linux, and then got redhat 8.0smp kernel update for athlon processors!
install kernel update fiorst then the driver. once thats done, get a toold called yanc 0.1.1
here is the link
just copy and paste that and down laod thetooland install.
once u have done that u should run that tool help u setup twinview, (you can goto http://yanc.sourceforge.net/
now edit your XFreeconfig file
look for "vesa" or "nv" in thedevice section rename it to "nvidia" and save and exit
now just restart x and viola !