external monitor resolution problem

[Nik N]

I have a problem in setting an external monitor to its full resolution
in 9.10 (64-bit). It worked in 9.04 (also 64-bit).

Hardware:
Acer TM 5530 laptop with ATI/AMD FGLRX graphics driver.
Samsung SyncMaster 204B (20", 1600x1200 native) external monitor.
External monitor is connected with a vga->dvi cable from VGA external
port to analog monitor input.

In 9.04 it took some 'fiddling' (multiple attempts, difficult to
reconstruct) but in the end the external monitor worked at its full
native resolution.

Using [Display Preferences] in 9.10 I can get the Samsung recognized
([Detect monitors]...), but when activated, its resolution list goes
indeed somewhat higher (up to 1440x900)  than the "primary" laptop
display (which is 1280x800). Oddly, on the expanded resolution list
for the external monitor there is a  couple of blank, "unpopulated"
rows above the 1440x900 row (not "grayed", simply blank) which can not
be selected. It appears that those line would, if populated, go up to
full resolution (1600x1200).

Questions:
Is there something obvious I'm missing?
Which X configuration file could be compared between two installs
(9.04 and 9.10) to offer some insight.
What would be the consequences of manual edit of such file?
Anything else I could try?


TIA, Nik N.