Hi, and thank you very much for the advice, druuna. However, I have this figured now. The problem was not the GPU. What I did not mention (reading my above posts) was that I also had a similar problem with Ubuntu a while back, when it got stuck in 640x480 mode, and that with an onboard Intel G41(R) GPU.
The real problem appears to be some glitch with the monitor auto detection by xserver-xorg. I have an Acer x193w. I have been reading around laconically during my free time, plus a couple of visits to the Debian IRC, and it seems that there are several LCD monitors that are not very good at "reporting" to the OS their capabilities, so to speak. In my case, specifically, it was the refresh/synch rates in the Monitor Section.
If you have a similar problem, look around on the web to find your LCD monitor specs and try something like this in your xorg.conf file (with your specs, of course)...
VendorName "Monitor Vendor"
ModelName "Monitor Model"
HorizSync 30.0 - 80.0
VertRefresh 55.0 - 75.0
..and maybe even this sort of thing, in the Screen section (again, try and include the native or recommended resolution for your monitor, in my case it is 1440x900)...
Modes "1440x900" "1024x768" "800x600"
It might help.
My screen looks brilliant now. Therte's nothing like using the recommended resolution.
As a side note, there is a woeful scarcity of comprehensive documentation for xorg configuration structure, at least as far as I can see, just snatches, like what I have written above. Does anyone know of some tutorial or a "syntax & format" style guide for xorg.conf configuration? It would help.
Of course, if you don't have an xorg.conf file in /etc/X11 after installing xserver, you can make one by running
as superuser. cp it from /root to /etc/X11 and edit away.