I'm currently building an NAS. I have 2 3TB hard disks. Is it possible to software RAID these two if they hold the operating system? Or do I need a separate OS disk? If yes, could I just use an USB Stick for that? Cause a 40€ 1TB hard disks IMHO is overkill just for a small Debian installation.
It's surely possible. RAID1 has the added advantage that even if GRUB doesn't know it exists, it can read one half of the mirror and boot the system like that - once it hands over to your initramfs mdadm should take over, assemble the array, and then the kernel can really kick in.
You'll want to look at mdadm - it's in the repos, so apt-get will work for that, and the debian kernel will handle it happily. You'll need a liveCD, but first install mdadm into your OS and then boot from the CD so you can build the array, let it sync the data to the other half of the mirror, update your fstab accordingly (you'll most likely now have / on
GRUB2 does come with a raid module you can tell it to load and boot the root from the md device, which might make more sense for a long-term solution.
It's also possible to run the OS from a USB stick, though it might be quite slow.