1) It does not matter how many slots you have free in your tower. Many (if not most) things that would kill One raid 5 would also destroy the second in the same tower (think of fire, cathastrophic PSU failure, theft).
2) This means you need a seconds box anyway, so I would either suggest a NAS, or a second computer with you backup to regularily on the network. (NO mirroring. If its transparent, its also transparent to errors and no backup).
3) no tabe. Reasons:
a) practiability: A robot seems to be way outside your budget, if you worry about slots in your case. You wouldnt like switching takes any more than switching HDs.
price: if you dont want to work yourself silly, you would need 800Gbyte tapes. The cheapest LTO-4 drives i see around here are a bit above 1500â‚¬. Add to this 50â‚¬ per tape (thats about the same price as a 750Gbyte HD).
For the money of even the cheapest tape drive, at current prices you could build a computer with 2 4TB raid 5s (2* 4 Seagate 1.5Tbyte raid 5, 2* highpoint 2310) to alternate snapshots. Just to give some perspective.
For me, i also have a big raid, but i do not consider it 1 piece of storage. I have snapshotted online-backup for the most important 20-40Gbyte, keep a 2nd copy of the the "would be a hassle to rescan/digitize" stuff on external HDs (1 Tbyte or so), and the rest is just temporary stuff (uncompressed datasets, evaluation stuff) that can be recreated in about the same time as restoring an external backup would take, so no point in it.