After many experiments, I final got Xorg configured in such a way that I could use both the optimized (but closed source) NVidia driver and the Nec 20wgx2's native resolution of 1680x1050!
I assume its the Option "ModeValidation" "NoMaxPClkCheck, NoEdidMaxPClkCheck" in the config file below that actually did the trick. It basically tells the driver to ignore whatever the videocard claims its pixel clock limitations are.
Obviously the videocard is not aware of its own power...
Technical info and keywords:
GeFroce FX 5200, AGP 8x, 128MB NVidia driver version 1.0 build 8762 Xorg version 7.0.0 (7.0.0-0ubuntu45) Ubuntu "Dapper Drake" 6.06 Nec 20wgx2 1680x1050 pixels
Some references that got me to try this:
http://www.jwdt.com/~paysan/dell2405fpw.html http://www.nvnews.net/vbulletin/showthread.php?t=52052#4 http://www.nvnews.net/vbulletin/showthread.php?p=633393
And see my current xorg.conf file...