RAID vs. Single Big SCSI Disk

We have three databases for our scientific research and are getting
close to filling our 12 Gig partition. My boss thinks that just getting
a really big (i.e. > 30 Gig) SCSI drive will be cheaper and should do
nicely. Currently, we only have 4 people accessing the database and
usually only have 1-2 jobs (e.g. selects, updates, etc.) going at any
one time (probably a high estimate). The db sits on a Pentium II/400 MHz
with RedHat 6.0.

Other than mirroring, are there any other advantages (e.g. speed, cost)
of just getting a RAID controller over, say, a 73 Gig Ultra SCSI Cheetah
drive (which cost in the neighborhood of $1300).

Also, can Postgres handle being spread over several disks? I'd think
that the RAID must control disk spanning, but just want to make sure
that Postgres would be compatible.

Thanks
-Tony Reina