OK I was fighting with this same issue...

I wound up using:
Code:
fb1280x1024 vsync=60 xmodule=nv
That boots with nv(fbdev) at 1280x1024. If you omit the xmodule it will boot using Xfree86(fbdev). Specifying the screen seems to be unnecessary, but...
w/ Knoppix 3.3_2003-09-24 you need to put a couple of spaces at the end of the line since it 'eats' the last char or 2. I originally had the vsync=60 at the end and noticed it was trying to use 6Hz!

Keywords: NVidia GeForce FX 5600 DVI LCD panel sync