We also started out using 2 7331 libraries with 8mm tape.  When we
outgrew that we went to an STK9714 with DLT7000 tapes.  The STK9714 can use
up to 6 drives, and up to 100 slots, so you can start small and expand as
you need to.  We had all the usual problems with the DLT tapes, but that
seems to be ironed out now, and the library is running well.  The DLT7000
media holds a good amount of data, and has good speed, and there is a newer
DLT scheduled to come out in a year or so, so there is also a migration
path for the meduim.  This combination would let you start small and grow
as needed.

        We use the STK9714 for our primary storage pool (along with a disk
pool), and then use our old 7331's for archive and offsite copy pools. 
This combination has worked well for us, and we have been able to continue
getting use out of our investment in the 7331's.