thats not possible. the method tha the xbox partitions the harddrive is hardcoded into the system. hence, at boot, the system will read only a disk formatted with what it recognizes.
further, from my research, ubuntu isnt a very good system to run on a standard issue, 64Mb 733Mhz xbox. the newest release of xdsl is beautiful compared to the older release, and is fast. runs great.
something about linux installs on your xbox, however:
no matter what linus you want to use, if you have a v1.6 xbox, you will *not* get useable video results. the proprietary video chipset on the v1.6 has no instructions released, so no drivers have been made for it, and never will be.