I dunno about 3.0 but in the old Debian Potato, before running startx, you've to first run:
If I remember it right. That brings a screen to configure the hole XFree as mouse, keyboard, monitor and stuff. When finished, that setup was the one who wrote the XFree86 file and then, only only then startx was valid. Things change fast in open source though, so it might be different on Debian 3.0, but take a look at something like that for configuring the Xfree...