I've had it running perfectly with RedHat 9, so I suppose it might work with you, though I haven't tried it with the G400 card, I had an Nvidia and I can't remember which one at the moment. I have the same problem as you now but with a bland Intel 845 build in graphics on a dell dimention 2400. This configuration should work with you though:
Section "Monitor"
Identifier "Monitor0"
VendorName "SGI"
ModelName "fpd1600sw"
HorizSync 31.0 - 74.0
VertRefresh 60.0 - 60.0
Option "dpms"
Modeline "1600x1024" 108 1600 1616 1656 1704 1024 1027 1030 1056 -Vsync -Hsync
EndSection
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1600x1024"
EndSubSection
EndSection
Section "DRI"
Group 0
Mode 0666
EndSection
good luck.
more configuration of this particular screen is available on my blog at
http://mahmood.tv/index.php/docs/1013