I am planning to switch from Ubuntu to Windows 7 for my HTPC and later add an extra 3TB disk.
Can I install Windows 7 on a 3TB disk and later add a disk and upgrade it to 2x3TB software raid1 without reinstalling the entire PC?
In theory: yes. Just create a RAID-1 array with one missing disk, then install the OS on it.
It will work fine (though it will complain about the array being in a degraded state).
I started with "In theory" because while I know this works on Linux/Ubuntu with mdadm, I have no idea how windows will handle that. Or if a hardware RAID controller will allow you to create a mirror with only one disk present. (Some brands/models will, some will refuse).