I've just built a new system, the motherboard has an inbuilt ATI 4290 video (part of AMD GX890) and I'm using it with a Dell 2405 monitor.
I've installed the latest Catalyst driver (10.6) from the AMD site, I can build the driver and X starts OK. The control panel for Catalyst says it will support 1920x1200 for a DVI connected screen - which mine is. When I start X it's always 1600x1200 and if I set the resolution via the Catalyst control panel, I get a display that has a black edge on the right of the monitor - about 1/4 of the screen.

At this point, I haven't tried setting any Modelines in xorg.conf as the fglrx driver is getting the information from the monitor via DDC and I'm assuming (probably a bad idea) that this data is correct.

Can anyone make any suggestions regarding this? Should I be setting Modelines regardless of what the driver gets from the monitor?
Log files available if requested

Other that the 1600x1200 limitation, the system (AMD X6 1090t) runs very well.

If you have an xorg.conf, you can stick a line in somewhere(don't ask me where)

Virtual xdim ydim
e.g. Virtual 1600 1200
man xorg.conf

I guess a better question should be:
Can/should the xorg.conf timing details override those obtained via DDC?

Up till now, I've only used the monitor via the d-sub connector i.e. analog.
I have tried connecting it via DVI to Mac Mini and it sorted out the resolution correctly to 1920x1200.

Yes, configuring is necessary. Take my example - no .ie keyboards are made! The focal length of my eyes is increasing with age, so I don't want tiny fonts