Your Tivoli Support Team should be able to answer this question at least in
AIX processor terms.  This is what I think:
        (1) AIX P660 4 processor with 2GB memory (equivalent) (2) if you
want to have good capacity.

What kind of database size do you have?  That is a key missing sizing
element here.  1 P660 could handle the amount of data you have if you use a
Gigabit network with dual Gigabit adapters and a limit of 200 simultaneous
sessions.  The key is the size of the database.  Also, like on VM/MVS you
can run multiple copies of the TSM server on AIX and probably Solaris.  Just
different ports for each one.

I have 1 P660 backing up a 8TB Windows server environment (clients), many
are 100mbit connected, 20 or so are gigabit.  My P660 is as above, but it is
connected to a Shark so that makes the database fly.  This all depends on
your database size and whether you want to run a multi-server configuration.
I am backing up about 300GB/night and the P660 isn't even doing anything
half the time.

The nice piece about SAN connected tape is it flies on copy pool copies of
large files and large file backups, but you have to have gigabit on the
network side to get the data there and your clients have to have high end
disk in them.

For the little amount of data you are backing up.  I would buy 500GB of
high-end disk (T3s are not high-end disk), scale back on the number servers,
run everything to disk pools, and use the tape drives only for migrations,
restores, and copy pools.