Re: Asus a8n-vm, nv8178, Dell 2005FPW
I have a 20'' display with the same native resolution (1680x1050) and I needed to specify an option to the nvidia drivers as well as a new modeline. the option is only required if you are plugging your monitor through DVI:
ModeLine "1680x1050" 145 1680 1784 1968 2256 1050 1051 1054 1087 -hsync -vsync
Option "ExactModeTimingsDVI" "true"
If you try using DVI with a widescreen resolution without this option it is more likely to fail and stop sending signal to the monitor.
Hope it helps, since I went desperated until I found out this.