These are some no doubt simple minded suggestions:
As a word of caution, I like to jump into cold water all at once instead of putting in one toe at a time. Having said that, I haven't tried ubuntu 7.1 setup on a raided system, however, there should be no harm in trying to see how smart the installer is, given a few precautions. BUT, backup your data and system before doing anything else.
The ubuntu installer will ask where to put things; if it doesn't look right (ie you don't know for sure that the disks it wants to install to are the ones you want to install to) - don't do it. Setup the system disks first, then set up raid 1 on the system disks (there's an excellent how-to on the ubuntu site): google "howto raid ubuntu" it's #1
If you don't let it touch the raid 5 disks, I doubt you'll have a problem rebuilding the raid 5 array after your new setup is complete.
good luck, let us know how it goes