Hello everybody,
this text is made by google translator, plz forgive me any errors:

"How is this possible? Here’s the setup: my "slowest" retro PC features a P166MMX, an ECS P5HX-A motherboard, a Spea Mirage V7 2MB PCI card, and an IDE2SD adapter with a 32GB SD card. It runs Quake 1 v1.08 in software mode at resolutions up to 360x480; anything higher results in a black screen. Just for fun, I pulled my Gigabyte GA-586HX (rev. 1.54) out of the closet to build a pure DOS PC as a test. I took my second Spea Mirage V7 card and installed it, along with the IDE2SD adapter and 32GB SD card from the ECS PC, just to see what it felt like to run DOS exclusively and control the P150 using SETMUL. Then I wondered what would happen if I launched Quake 1. I thought, "Hey, why not try changing the resolution?" since it always starts at 320x200. I "accidentally" selected 640x480, expecting a blank screen like usual—but to my surprise, I actually got an image. I was amazed. I bumped it up to 800x600, and the image remained visible. It wasn't until I hit 1024x768 that the screen went blank. I figured the Spea Mirage V7 from the ECS PC might be faulty, so I swapped the cards. The result was the same: the ECS PC showed a blank screen at resolutions above 360x480, while the Gigabyte PC handled up to 800x600. Then I swapped the monitors—replacing the Asus VS247HR with the "EyeCare" model I had been using to test the Gigabyte PC. It’s the same story again: the ECS + Spea combo gives a blank screen at resolutions above 360x480, whereas the Gigabyte + Spea setup displays an image up to 800x600. How is that possible? Both boards use the Intel i430HX chipset. I’m also running the S3 VBE driver in autoexec.bat—the one I found on theretroweb. By the way, testing with Duke Nukem 3D yields the same result; on the Gigabyte board, I can use VESA modes up to 800x600 with the Spea card."

EDIT: both Spea V7 deliver a picture to 800x600 with both monitors on my Gigabyte Board whereas both remain blank on the ECS Board.

EDIT2: the ECS has a Jan Steunebrink Bios, which supports K6-2, K6-3, 128GB HDD whereas the Gigabyte has the last Peter Sucek 3.0p Bios in order to support K6-2, K6-3 and 128GB HDD.