Cheatcodes can be a tricky thing on picky hardware, you just have to keep at it and eventually you'll find the right set. At least it boots!
You could try something like:
knoppix xmodule=s3 screen=1024x768 pci=biosirq vsync=60
or
fb1024x768 xmodule=s3 pci=biosirq vsync=60
Throw in any no-options you feel like testing, sorry but it's kind of hard to guess, it's one of those trial and error/success things.

EDIT: BTW the vsync thingy stands for vertical refresh rate which is what your monitor was complaining about with the "out of sync" error. 60 will flicker quite a lot if you don't have LCD but is the best that old monitors will do, so you might want to check the manual for your monitor and videocard to see what they support for different resolutions.