I think Miles gave a great answer, that is just the approach I would take.

We are currently running 500 clients on a dedicated F50, 2 processors, 1 GB
memory, 4 STK 9840 tape drives in an STK 9710 robot.  TSM is at 4.1.3
2 network adapters, one GB ethernet, one 100MB ethermet.

Clients are primarly Win2K & WinNT; some Win95, Win98, AIX, OS/2, MAC.
We back up about 140 GB a day; 10 GB is from one data base, the rest is all
small stuff.
The small stuff actually has more impact on your server than big chunks of
data from data bases.
Total storage is 5 TB on site, 5 TB offsite.

The DB is 30 GB, 82% full.  Currently have 2 buses for the 9840's.
When the load was smaller I had the DB on RAID; had to give that up to get
more speed and go to physical mirrors.
Disk is some SCSI-RAID in an external RAID cabinet (slow, used only for
storage pool), SSA disk, internal Ultra SCSI disk - essentially anything I
can get my hands on, spread across 4 adapters.  Total of 130 GB in the disk
storage pool.

The good news - as a former mainframe storage and performance person, I can
tell you that you will be very pleased with the performance you get out of
AIX - I don't think you will have any problem at all, the I/O just screams.
TSM isn't actually very CPU-intensive when it comes to backups.  Our F50 is
just cruising along most of the time.  We're getting a bit tight on time
schedules, but the problem is reclaims, and expiration, not backups
themselves.  Again this is because we do 130 GB of small files every day.
If I had a bigger robot, and could reclaim less often, there would be no
issue at all.

Miles is right I think, in that for a system your size you should be
concerned about I/O more than anything.  More good news - you will be able
to afford a lot more disk on an AIX box.  Our last buy was for 3 - 18 GB
drives, less than $5K.

Remember that, just like on OS/390, bigger drives aren't necessarily always
better- make sure you buy enough disks  (not just enough space) to
physically mirror your DB (preferably across different adapters).  Make sure
the RS6000 you buy is expandable so you can add adapters, as well as disk,
if you need to spread your I/O some more.

Good luck!
 ************************************************************************
Wanda Prather
The Johns Hopkins Applied Physics Lab
443-778-8769
wanda_prather AT jhuapl DOT edu

"Intelligence has much less practical application than you'd think" -
Scott Adams/Dilbert
************************************************************************