Jaunty on Dell Desktop

My Dell Desktop is old -- I mean very old. It's a Dimension 4550 from 2002 that came with 256 MB of RAM (since upgraded to 1 GB which is the max) and a nVidia GeForce4 MX420 graphics card and 80 GB of HD.
It's been running with Ubuntu Hardy for quite some time without any problems. But today I decided to bring it up to date.
So, first I tried the network update to Ibex (8.10) and this seemed to go fine. However, when I rebooted to see how it looked, I ended up with a completely white screen. I'd seen this before but couldn't remember where and what I did to get over it.
Anyway, I next tried the CD install and this went perfectly .. absolutely no problems, well, almost.
I've now got all the usual stuff installed, Opera 10, Yakuake, 3D Effects, AWN (although I might replace this with Kiba-Dock as it;s more "exciting").
The one problem I have is that, although I can change the screen resolution to 1280x1024 (60Hz) using the nVidia Settings Manager, the /etc/X11/xorg.conf doesn't seem to get updated.
As a result, when I reboot, the resolution comes back as 1024x760.
Really don't understand this.
I tried adong the line
Modes "1280x1024" "1024x760"
to the screen section of the xorg.conf file but this gave a boot error which said that it was only now able to run in low resolution mode.
Strange. Need to look into this but I have a feeling that my ancient graphics card may be showing its age.