In this particular case, you should be able to install your Operating System in your 80GB drive. This way, you would need to configure your 80GB disk as a standalone RAID 0 drive for your Operating System to see the drive during the installation. You would still need to load your RAID driver to install the operating system.
It is not required for the bootable volume to be a RAID 0 or RAID 1 volume. As a matter of a fact, you should be able to create both volumes at the same time and then install the operating system. And yes, you need to get the RAID Activation Key (AXXRAKSW5) to enable RAID 5 support.