HDTV native resolution issue

[Grant]

I just set up my new 32" LCD HDTV (Hyundai Q321).  It has a native
resolution of 1366x768, but I can't get my i810 onboard video to
output at that resolution.  If I specify:

DefaultDepth 16
   SubSection "Display"
   Depth 16
   Modes "1366x768"
EndSubSection

and startxfce4, the screen just goes black.  In a previous thread
here, someone thought the i810 had a set list of modes in BIOS, but
the conclusion was that it could set modelines itself.

Xorg.0.log says:

(II) I810(0): Not using mode "1366x768" (no mode of this name)
(--) I810(0): Virtual size is 1600x1200 (pitch 1600)

If the TV's native resolution is 1366x768, it can't go up to 1600x1200
right?  Maybe that's why the screen just goes black?  X starts at
1024x768 just fine, but I'd really like to run at native 1366x768.

- Grant