OK, when I use the "expert" code, it still says undefined mode number, but I skip it and get to this:

Video is GD754x (laptop) GD 7548, using XF86_SVGA(vesa) Server
Monitor is Generic Monitor, H:28.0-96.0kHz, V:50.0-62.0Hz
Using Modes "1024x768" "800x600" "640x480"

after I say yes to run XF86 config, I set it to supervga 800x600 for the monitor, the lowest refresh rates, choose my GD7548 for the card, choose vesa, and then I get strange lines and blurry shapes on screen.

same thing happens if I choose "no" to reconfiguring X11.