I just tried my brand new Acer VW257 (1920x1200) on my FX 5900 and either get an out of range message with the FX 5900 as primary in Windows 98 SE , or Nvidia driver crashes with FX 5900 set as secondary while trying to set resolutions .

The root cause of the problem was the Nvidia driver's dislike for the monitor's EDID (the monitor is 75Hz capable at its native resolution). This is not due to the 1920x1200 resolution as such, since a Dell U2412M (1920x1200 at 60Hz max) worked fine in this setup .

I was able to resolve the issue by cloning the EDID of a Dell 2007FPB using a Siig EDID emulator model CE-H24Z11-S1 , which has cloning capability . My intent was to run it in 1600x1200 anyway, so this is perfect for me .

What if I wanted 1920x1200 or if I simply did not have access to a monitor to dump EDID from ?
Siig does not provide software to program this device from a PC, after all .
Well here's a fun fact . The Siig EDID emulator model CE-H24Z11-S1 is practically identical in external appearance to the Gofanco Prophecy EDID emulator . Gofanco does provide software for its device and the software recognizes the Siig CE-H24Z11-S1 . The software allows reading (dumping) and writing (from a file) an EDID over USB from a PC . There's even a simple EDID creator tool, though I would use Analog Devices Advantiv EEditZ for editing EDID files instead .
EDIT: I am not 100% sure writing an EDID works. I modified an EDID file to try to see if I could get 70Hz at 1600x1200 with reduced blanking and "wrote" it to the device, but saw no changes . Dumping EDID from a connected monitor definitely does work .