X and Fluxbox :: VGA mode with 56Hz

That, I don't know. I do know that you can start DSL with vga=
codes at the boot prompt at first startup. You have to be quick,or
DSL will boot with a default 791 code.
I use these two:
boot: knoppix vga=789   (gives 800x600 at 32 bpp)
boot: knoppix vga=787    (gives 800x600 at 16 bpp)
you can try  these, one at a time:
769   8bpp  640x480
771     "      800x600
From what you have, I doubt that computer can do 32 bpp,
but if it can, try 786 for 640x480
or if you want to have 1024x768, use 792