Here are my thoughts, but I think you should talk to all the vendors you can as this is physically going to be difficult to achieve due to the sheer size of the DB.
If this database is critical, or even remotely important to the continuation of your business, then you should consider an online mirror, along with a remote replicated mirror.
You can stop there, but to offer additional protection against corruption, etc., you are going to have to do some sort of snap to some disk storage. You won't be able to protect the database with a traditional backup as it will blow your RPO out significantly. Once it is snapped to disk, you then need to decide if you want any other copies. You can copy the snaps and/or the online mirror to tape and keep them as long as needed to meet your business requirements and any governing regulations. Tape is going to be faster than disk backups in this case and it will be significantly cheaper.
Either way you slice it, to do it right will not be cheap and you will need to do a lot of testing, both on the backup and recovery side to ensure it fits within the business' RPO/RTO requirements. Don't forget to continue testing recoveries on a scheduled basis, especially after upgrades.
Good luck. If you can remember, I'd like to hear what your end solution is.
-Rusty
