File system organisation

We're in the process of buying a 2TB NAS (RAID 5) for the office. We've currently got around 150gb of work in progress files and 300gb of archive, growing at around 1/2gb per week. We've got a tape drive backup but it's only 80/160gb so requires tape swaps to backup completely. In addition, we also perform backups to a 500gb USB drive throughout the week and then a full backup to tape.

I'm looking at the best way to organise our filesystem when the new device arrives. Part of me thinks, split the 2TB down the middle and have one share for work in progress and one share for backup, then backup everything to the 500gb USB drive and run an offsite tape backup once a week as normal.

Alternatively, I could suggest a replacement tape drive big enough to backup the NAS. Any suggestions?