"I just saw, that a 'downgrade' of the bios is not necessary anymore
You can also upgrade to A04. It works fine!
Dell support page writes:
The following changes have been made to the A03 BIOS to create the A04:
1. Fixed 1M/8M switch for non-Windows OS.
2. ..."
Yup, I can verify that. Upgraded to BIOS A04 (on an Optiplex GX270) and set the AGP aperture to 8 MB (it's set to 1 MB by default.)
Currently viewing my new RedHat 9 install through a 1280x1024, 24-bit color porthole. And it looks much better than Windows.