Only getting low resolution with new monitor in feisty

[D. R. Evans]

I just installed a better LCD monitor on my wife's system.

Unfortunately, although Windows has quite happily reset the resolution
to 1280x1024, feisty keeps insisting that X is producing an error
whenever I press "Test" in the KDE configuration tool.

I installed the binary ATI driver, but that made no difference

Details:

Graphics card (according to KDE Monitor & Display | System Settings):
ATI Radeon (fglrx)
Monitor is a ViewSonic VX724

When I run the Test after trying to set the mode to be 1280x1024
(60Hz), I get the following, no matter what I do:

----

Sorry, this configuration video card driver
and monitor doesn't appear to work.

Messages from the X server:
Fatal server error:

----

(No there's no cut-and-paste error there, the actual 'Fatal server
error' is not displayed.)

Since this is my wife's first interaction with Linux, she is
understandably unimpressed.

Any idea what I need to do to fix this?