Nice as branded hardware is, it can be hella expensive!

..and I looked at the HP offering, no small amount of the cost of that is paying for Windoze 2003 Storage Server (which further raises the question of licencing, CALs and all that (seeing as it always seems to come up with M$ stuff)).
What's the model number of the one you have? ..while you may not be able to get from 18GB (80pin SCSI, yes?) disks (and do you REALLY want to buy 146GB SCSI disks??!) it may be possible to get you something else that'd still get you more space than you have now...