X and Fluxbox :: How do I use a nonstandard resolution?

I have a laptop with a screen resolution of 800x480. Everything works, but I'd like to be able to use the whole screen. Right now, it's like a reverse-letterbox screen, with black bars on the sides.

cheat code at boot is "dsl xsetup"
From a running system do ctrl+alt+backspace then "xsetup.sh"

DSL uses the lightweight Xvesa X-server and this (I think) only supports the standard resolutions. You probably have to install and configure the full XFree myDSL package.

non standard resolutions

original here.