XFree86 Configuration

I am having trouble configuring XFree86 correctly for my monitor. I am using Mandrake 9.0 with XFree86 4.2.1. I have a GeForce 3 Ti200 64MB and my monitor is a Phillips 15C. I don't expect anyone to have the same setup that I do although....you never know!
When I try to run the desktop at 800X600 the screen is way over to the right....by almost 30% of the screen! I've used xvidtune to centre the screen and get the correct settings string but when I enter this into /etc/X11/XF86Config and restart the same problem appears. I have deleted all other mode entries but the same occurs each time.
Can anyone offer any advice?

Don't you have to adjust that on the screen. I don't know how old a Philips 15C is, but it has to have centering controls, right?