We use Compaq DLT 40/80 GB tape drives here at work. They are nice and fast and do the job well. They have 40 GB native capacity, so if you think your data will compress 2:1 then this might be a good option (or even if not, it will still only take 2 tapes). Another option is a SuperDLT drive by Quantum (I think). It has a 110GB native capacity, but likely VERY expensive.
If your data is very compressable, you might get away with a 20 GB native drive, the main concern is that you don't really want to have to change tapes too often (if you are around) or at all (if you run the backup at night when no one is around).
Hope this helps...