I'm running Debian Etch on an old thinkpad i1400, but the maximum resolution available in the Gnome screen res dialog is 800x600. The display is capable of 1024x768. I went through the xorg.conf file and removed all the 800x600 and 600x480 modes, but that didn't work. I looked through the xorg.log file and found that the neomagic driver says that there is not enough memory for 1024x768 and over. This is, of course, not true. I have Windows XP running on the same system and it displays 1024x768 just fine! Is there some way to force X to use the propper resolution, no matter what it thinks?

I think that you adjusted nothing regarding the type of Monitor.
Thats why Ubuntu (I am not a Ubuntu man :P ) but it's standard that x configures
a 800x600 Monitor so you will never be able to drive a screen resolution of 1024x768 of course. So what you have to do is just to choose a huger LCD Display type.
In your /etc/X11/xorg.conf the important section is Section "Monitor"

so maybe this xorg.conf will fit your needs:

Section "Monitor"

### Comment all HorizSync and VertSync values to use DDC:
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "Dell 1024x768 Laptop Display Panel"
### Comment all HorizSync and VertSync values to use DDC:
HorizSync 31.5 - 48.5
VertRefresh 59.0 - 75.0
Option "dpms"
EndSection

Section "Device"
Identifier "Videocard0"
Driver "i810"
VendorName "Videocard vendor"
BoardName "Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 16
Modes "800x600" "640x480"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection