Serious problems with NVIDIA FX5500 + Win98SE

Hello!

I'm running an older PC with Win98SE and SuSE Linux 10.1 in dual-boot (which
worked fine without any serious issues so far). The configuration is:
- AMD Athlon XP 1900+
- Gigabyte GA-7VTXE+ Mainboard with Via chipset
- 1GB RAM
- AGP 2.0 Slot (AGP 4x)

Now, unfortunately my old graphics card (GeForce 3 Ti200, 64MB) completely
passed away (the computer kept just beeping), so I bought a GeForce FX5500
with 128MB. There were no problems installing the card under Linux, the
latest nVidia driver works fine (except for offering me only 75Hz (?) :-( ).

However, under Win98SE, after changing the card I had no chance making the OS
work even with the most recent Win9X-drivers (first Forceware V77.72, then
V81.98). I have now wiped the whole partition and reinstalled Win98SE with
the most recent available Win9X-drivers for the chipset (first tried VIA
4-in-1 V4.43, then went on to VIA HyperionPro V5.10a), since the drivers
originally shipped with the mainboard didn't work either. I have also tried
DirectX 8.1 and 9.0c.
During all my attempts to get Win98SE running, the behavior is strange and
unpredictable with these effects:
- Unexpected halts with black screen while starting Windows
- Message boxes saying that the graphics settings were incorrect ("Either
graphics settings are incorrect or hardware doesn't support the settings")
- Windows temporarily freezing when launching nwiz.exe (killing this task
at least brings up the desktop)
- Windows falling back to 640x480x4 and not letting change the video mode
althouth the FX5500 is set as graphics card
- dxdiag
- etc.
And, after the next machine reboot, the effects may be quite different (no
special order).
I temporarily managed to get one installation running with the Via Hyperion,
DirectX 8.1, and Forceware 77.72 (Direct3D tests worked fine, I could also
change to any resolution). However, after rebooting it didn't work anymore -
the effects listed above occurred. I had also made a partition image of this
"working installation", which, when I restored it, also didn't work anymore -
yet it had worked before once.
The BIOS setting for the AGP aperture size was 128MB for the last years, but
changing it to 64MB didn't help anything as well.

Does anyone have an idea of what's wrong and what to do with the windows
installation? I preferably want to keep Win98SE, because I run some older
software and games. Any hints will be greatly appreciated! I have now spent a
couple of days installing and have run out of ideas...

Thanks in advance!

Andreas