widescreen resolution with vesa driver

I used to read that vesa doesn't support "1680x1050".
However in FreeBSD handbook a trick is described with a sample exactly for this resolution.
In general a "ModeLine" line to insert in "Monitor" section:
ModeLine <name> <clock> <4 horiz. timings> <4 vert. timings>
and the sample:
ModeLine "1680x1050" 146.2 1680 1784 1960 2240 1050 1053 1059 1089

According to the handbook the clock and timings values could be retrieved from /var/log/xorg.0.log
Unfortunately I couldn't find such a data in my xorg.0.log instead I found
"1680x1050" - not used, no such name
Well, I took a risk and inserted the example line "as is" into my Monitor section. As a result I got Gnome working @ "1680x1050" - beautiful!
On the other hand KDE4.1.0 doesn't want to work with this configuration: it just restarts back to login screen. As I guess the timings are not exactly what my monitor requires.
The manufacturer's manual and specifications don't include the timings values. How to find them? Is any proggy in ports available to get those values from my monitor?
Asus M3N78-VM, AthlonX2 (4450e) 2.3GHz, DDR2-800-4GB, Nvidia chip , GeForce8200(onboard), SyncMaster 2253BW, FreeBSD 8.0-CURRENT-200812-amd64.