Bro - if you are sure you have the correct specs for your monitor, you should use them in you XF86Config file. If the image doesn't fill the screen perfectly, maybe the problem is just that you need to use the adjustment buttons on the front of the monitor itself to adjust the image size, etc.
I ran into problems myself where the screen would go black during the boot up. The problem was that is was specifying the wrong video card settings, not the monitor. (Basically, I just need to set the default color depth to 16 from 24, and specify that I had only 64 Mg of VRAM.) My advice is to start off slowly, with low-end settings, and assuming that you do get a picture on your monitor, to incrementally tackle each problem one at a time, ie, start with a basic 800x600 resolution, then move it up to 1024x768, etc.
My main point though is just that perhaps your problem may be easily solved by using the monitor adjustment controls rather than being a setup or config problem. -- J.W.