I'd put extra as /media/extra (it will at least show up in sysinfo

)
I guess there are no problems in putting both RAID 0 and RAID 1 on the same disk(at least linux kernel is flexible enough to handle that). The other thing is putting /boot on RAID, my informations may be out of date but it is not possible to have /boot on raid since it won't boot? Or is that case valid only with /boot on RAID-0?