[CentOS] Problem with widescreen display

I have a dual boot CentOS 5.2 / FC4 machine, and recently I have bought a new 
widescreen tft monitor. I used to use a plain 4:3 crt, and after plugging the 
16:9 tft naturally X needed reconfiguring. This was easy in FC4, and seemed 
as easy in CentOS, but with a wrong result.

Basically, what I did was to run system-config-display to reconfigure for the 
new monitor and resolution. All goes well, but after X restarts, I see a 
strange picture: the resolution indeed goes to 1680x1050 as is supposed to, 
but is squeezed/shrinked/scaled horizontally to match a 4:3 aspect ratio, 
leaving two (unequal) black bands on the left and right side of the monitor.

This is specific to 1680x1050 resolution, while lower ones display ok up to 
the fact that the virtual screen is usually bigger than the displayed part so 
scrolling is necessarry (and this is annoying, for I cannot see the panel and 
the top of the window simultaneously).

The very same hardware and virtually same X configuration work perfectly ok on 
FC4, which suggests that this is not a hardware problem, nor an X problem. 
Further, as I see, FC4 has older version of virtually all software than 
CentOS.

I have tried various acrobatics with xorg.conf, but nothing helped; read 
Xorg.0.log inside out and back, compared to FC4, and everything seems 
essentially identical. X seems to work as everything is ok, so is mplayer 
(even in fullscreen), but the black bands remain there and the whole desktop 
is scaled to 4:3. The monitor "autoadjust" button also doesn't help (although 
it works in general).

I'm out of ideas where to look for the cause of this. If you wish, I can post 
xorg.conf and log files from both OSes, but they are mainly identical and I 
see nothing suspicious.

Btw, this is on an nVidia GeForce 4 using the default nv driver. The vesa 
driver doesn't support widescreen resolutions, while nvidia binary driver 
crashes X completely on start (but this is a known motherboard problem common 
to FC4 as well).

Is there some kernel setting or whatever that might "force" the graphics card 
to 4:3 aspect irrespective of X configuration? Some "filter" between what X 
tries to display and the actual signal to the monitor? What else can I try?

Any advice appreciated!

Best, :-)
Marko