EDIT: Deleted duplicate post.
Usually, you can check the file /var/log/XFree86.log (I think that is the name) for errors.
You have an NForce motherboard, right?
Did you downloaded both the NForce chipset drivers and the NVidia drivers for it? (They are 2 separate downloads. The nforce drivers give you sound and network drivers. The Nvidia video driver gives you ...a video driver, duh. )
This tells you how to install it.