The USB wifi device is a Ralink rt2750 device. It is supported by the rt2x00 driver family, and in etch you can just follow these steps:

cd $(mktemp -d)
m-a -u . prepare
m-a -u . get rt2570
m-a -u . build rt2570
sudo dpkg -i rt2570-modules-$(uname -r)*deb

In order to have a predictable network name to write rules for, I added the following to /etc/udev/rules.d/z25_persistent-net.rules:

SUBSYSTEM=="net", \
DRIVERS=="rt2570", \
KERNEL=="eth[0-9]*", \

If anyone knows where this device is, physically inside the computer, I'd love to know :) I'd probably put it in something else.