I'm currently at the point of trying to make Linux and RoS fit on the 2GB SD Card. According to a page that a friend found (and lost before I could document it), a stripped-down RoS install weighs about 800Mb. We may need a bigger SD Card; I can fit Linux on 1GB but it won't be pretty, nor will it have the bells and whistles we might expect. Next I'm going to try
debootstrap-ing an arm system and building up, rather than starting with a bloated Ubuntu image and paring down.
http://johnwoconnor.blogspot.com/2009/04/installing-ubuntu-on-gumstix-overo.html Installing Ubuntu on Overo: I used this site for information on formatting the SD card and extracting the image. Note the strange formatting for the SD card, Overo apparently won't read it any other way.
http://www.ros.org/wiki/gumros/ubuntu Installing RoS: A helpful guide for installing RoS. I haven't gotten there yet, so I can't vouch for its accuracy.
https://wiki.ubuntu.com/ARM/RootfsFromScratch ARM RootFSfromScratch: Very helpful guide for installing Ubuntu (and, to a lesser extent, debian) on an ARM system. Also includes a pre-built image (which works) at the bottom of the page.