I want to avoid that since the PCI card is pretty crappy and i hate to be changing manually the VGA connection and BIOS video priority each time I change the OS.

You can select the main PCI card in the BIOS (For the needs of Windows 98), and in Windows XP install the VBEMP for NT (Universal Video Driver) card on the PCI card, then add the GeForce 6600 and install the proprietary driver from the card manufacturer (or the reference from nVidia.com), go to the "display properties" settings and select the main GeForce 6600 there. So in Windows 98 you will have a PCI card by default, and in Windows XP you will have a GeForce 6600.

GPU: GeForce 6600LE

GeForce 6600LE PCI-E is not the best choice for Direct3D under Windows 98, since GPU support for this card was added to the drivers too late, and the latest more or less normal drivers for working with Direct3D are ForceWare 77.72, so, of course, try to add your video card ID to this driver and install. If it doesn't work, consider replacing your GeForce 6600LE PCI-E with a GeForce 7800GT\GTX PCI-E (G70 GPU based) or GeForce 6800 GT\ULTRA PCI-E (NV45 GPU based). Both of these chips are fully compatible with at least ForceWare 77.72

my Geforce 6600LE work only for windowed apps or Direct3d crashes often

Can we somehow reproduce these failures? What games do they appear in?

As far as switching cables, you can use a KVM switch or even some kind of branched cable if the monitor has only one input.
Something like this:

DoZator wrote on 2022-09-17, 10:31:

You can select the main PCI card in the BIOS (For the needs of Windows 98), and in Windows XP install the VBEMP for NT (Universa […]
Show full quote

I want to avoid that since the PCI card is pretty crappy and i hate to be changing manually the VGA connection and BIOS video priority each time I change the OS.

You can select the main PCI card in the BIOS (For the needs of Windows 98), and in Windows XP install the VBEMP for NT (Universal Video Driver) card on the PCI card, then add the GeForce 6600 and install the proprietary driver from the card manufacturer (or the reference from nVidia.com), go to the "display properties" settings and select the main GeForce 6600 there. So in Windows 98 you will have a PCI card by default, and in Windows XP you will have a GeForce 6600.

GPU: GeForce 6600LE

GeForce 6600LE PCI-E is not the best choice for Direct3D under Windows 98, since GPU support for this card was added to the drivers too late, and the latest more or less normal drivers for working with Direct3D are ForceWare 77.72, so, of course, try to add your video card ID to this driver and install. If it doesn't work, consider replacing your GeForce 6600LE PCI-E with a GeForce 7800GT\GTX PCI-E (G70 GPU based) or GeForce 6800 GT\ULTRA PCI-E (NV45 GPU based). Both of these chips are fully compatible with at least ForceWare 77.72

my Geforce 6600LE work only for windowed apps or Direct3d crashes often

Can we somehow reproduce these failures? What games do they appear in?

Thanks for your help!
Well, I've tried driver 82.69 from http://windows98.xf.cz/ and when I run dxdiag tests go fine until Direct3d fullscreen test which immediately crashes.
If I run for example Mortal Kombat 4, it runs fine windowed But it refuses to go fullscreen.

I have the 77.72 driver but I've never added a video card to a driver so I don't know exactly how to do it.
I've read that I have to add some text on the inf.files but that's it.

The last few Nvidia drivers for Windows 98 aren't that great even for older cards. Those drivers have a lot of stability issues. That's why I use a Radeon x800 with Windows 98. The last ATI drivers for Windows 98 are very stable.

I got a 6200 to work with this driver modification of 81.98 PCIe devices on Windows 98 SE

It works perfectly, on the thread it says 6600 is supported too but I haven’t tried. I had zero stability issues with it, no crashes. That said, you will have issues with certain games with a driver version that high.

fosterwj03 wrote on 2022-10-13, 00:27:

The last few Nvidia drivers for Windows 98 aren't that great even for older cards. Those drivers have a lot of stability issues. That's why I use a Radeon x800 with Windows 98. The last ATI drivers for Windows 98 are very stable.

Yeah, I read a lot about those radeon cards and I also think they are the way to go for a build like this.
I just wanted to re-build my 2004 gaming rig and add win98 to it so that's why I tried this build.

Zeerex wrote on 2022-10-13, 00:59:

I got a 6200 to work with this driver modification of 81.98 PCIe devices on Windows 98 SE

It works perfectly, on the thread it says 6600 is supported too but I haven’t tried. I had zero stability issues with it, no crashes. That said, you will have issues with certain games with a driver version that high.

Thanks a lot, but looking into it I see there's no driver for 6600 LE 🙁

And unfortunately I read someone said "SE or LE variants might have a 64-bit memory interface. Best to avoid those." So I wonder if that could be the issue that I'll write below:

UPDATE:
I was happy for a few minutes because I though I found the issue. Once I installed Directx 8.1 I was able to play Mortal Kombat 4 at fullscreen on direct3d and even run 3dmark99 and 01 without any issues.
This was done with unnofficial drivers 82.69
Unfortunately once I tried FIFA 2000 the game doesn't stop crashing after a few minutes playing, so this tells me that this is Not working 100%
I even tried drivers 77.72, 81.85 and 81.98 and the same happens.

I don't know if replacing the 6600 LE for a 6600 vanilla version could be the solution.