Re: FX 5200 DVI: No valid modes for "1680x1050"; removing.
Better to use cvt (I do not have the link with me) than gtf, since it allows reduced blanking modeline, which fits the 135MHz constrain of the 5200 DVI at 60Hz.
cvt 1680 1050 60 -r
# 1680x1050 @ 60.00 Hz Reduced Blank (CVT)
# field rate 59.88 Hz; hsync: 64.67 kHz; pclk: 119.00 MHz
Modeline "1680x1050_60.00_rb" 119.00 1680 1728 1760 1840 1050 1053 1059 1080 +HSync -Vsync
This here works for my LCD.
It might be the driver is really willing to support only full-blanking, while the monitor will accept reduced one.
There are several options of the driver in order to force the modeline, for example, in section "screen" of xorg.conf (or equivalent), you could try:
option "ExactModeTimingsDVI" "True"
Check the nVidia readme...
Hope this helps.