Can't get monitor to work at native resolution - Windows 7

I have clean-installed Windows 7 RC and the latest Catalyst drivers for my
HD4850 card. My monitor is a Samsung SyncMaster 2243NW, a 22" panel with a
native res of 1680 x 1050. I have installed the monitor driver from
Samsung's website. The monitor shows up correctly in device manager.

When I right click on the desktop and choose "screen resolution", the basic
setup screen doesn't show my monitor and doesn't offer 1680 x 1050. In
advanced setup, under the "monitor" tab, the monitor is identified
correctly. In the "adapter" tab if I click "list all modes", 1680 x 1050
isn't shown.

Installing CCC just makes things worse, as the monitor isn't even recognised
any more. Windows 7's own drivers for the card are worse, too. The Vista
drivers, unsurprisingly, are exactly the same.

Anybody got any idea what's going on here and how I can solve the problem?
The screen don't look good at 1440 x 900!