Re: FX 5200 DVI: No valid modes for "1680x1050"; removing.
My monitor is an LCD one connected using DVI (is there any other connection for LCDs... ;-))
The 135MHz limit is the DVI interface capability (monitor and/or 5200).
I understood we are talking about LCDs, these are less sensitive (or no sensitive at all) in case of frequency mismatch. Usually they'll not display anything or only a warning message, in case of problems.
In any case, the "-r" option of cvt is used to _reduce_ the frequency below the 135MHz limit.
Specifically, in the modeline I posted, the clock is 119MHz, which should be acceptable by all devices.
The only problem _could_ be in case the monitor does not like the reduced blanking, but if it does, then it's a dumb one... :-)
Note that modes like 1920x1200 _must_ have reduced blanking or dual-link DVI in order to work properly (below 165MHz).