Hosting solution for large storage

In an upcoming project, I will need to host a web server to potentially
serve users totalling 5 TB of data. As I have never worked with
anything in this scale, I am wondering if anyone has any
recommendations of cost effective yet scalable storage solutions.

So far I have looked into various things, such as SAN or simple
external storage - I myself am an Apple user for workstations, but have
always used Linux for servers. In this case, I have looked at both some
IBM storage solutions as well as the Apple XServe RAID solutions - and
accompanying XServe servers. Apple's XServe RAID offerings appear quite
reasonable.

Does anyone have any comments or recommended setups? If I start with
external storage (non-SAN), but then decide to migrate to SAN later,
will this be a pain? To start with SAN though (getting a fibre router +
the software) would seem significantly more expensive... but is it
worth it? I don't really want to budget more than $20,000 for the
computing infrastructure (including two servers + storage network +
tape backup). So far i've managed to do this without SAN.

Is anyone familiar with Apple servers at all? Any problems encountered,
with G5 compatibility for example? One good thing is that it "just
works"... whereas i've had so much hassle in the past with Linux, when
things break for no reason (usually thanks to some non-user minded
developer who experiments with live ammunition and doesn't notify
people - i'm talking the software packages here), or are just generally
a pain to get working.

Thanks for any comments!

Terence