There are plenty of guides for converting your Sheevaplug to boot from SD card. See
here for starters (lots of links). Frankly, it is much simpler to start with a fresh O/S on an SD card (and probably better to go with Debian than Ubuntu).
Most methods of booting from SD card rewrite the NAND with a fresh O/S install, so if you have essential data on your NAND you must make sure that you make good backups first.