Thanks for your replies

12 GB?  Hmm..

So some quick math makes it look like 12884901888 bytes/24 hours or
536870912/ hour or .5 GB per hour.

500MB / 1500 users, would be about 33MB per user per hour (I'm sure there's
plenty of other traffic; just generalizing).

Really?  What kind of files do they send around typically?  What is the
average message size that you deal with a) from the external connections and
b) internally?

It makes a difference as to the layout of disk/application and horsepower
needed to process it.

As an example, the smaller/higher volume messages going by (say 50K msg size
at a million per 24 hours) would require one particular type of disk
configuration to handle without bottleneck, vs. larger/smaller volume
messages (one that has to handle 10MB msg size at 50,000 per 24 hours.)

Then there's the client usage patterns.  Do all users use MAPI clients (you
answered that; 300 will use OWA via FE servers, but are there other client
types?  POP/IMAP?)  Do they all logon and use mail during the same hours?
Or are they spread out and will use them at different types of the day?
Maybe there are intersection points during the traffic when all users are on
but most of the time there's only 50% using at any given time?  This all
makes a difference if you want to cut it close.

Likely you have enough horsepower in terms of processor/memory/network.
1500 users with 300 of them being OWA is usually not that bad for just
Exchange.  If you plan to add other apps, you may want to be a little more
careful and test it more thoroughly to be sure it can handle the load.  The
wildcard here is the disk layout and types of traffic patterns/sizes of
messages. If those calculations are correct, you'll put a lot more strain on
the storage subsystem than just about any other part of the system (network
will have to handle as well though right?).  You'll want to pay special
attention to the spindle/application layout. Generally smaller/faster
spindles are going to be better vs. the 300GB spindles they sell with the
MSA1000.

Personally?  My instinct tells me it's possible to do pretty comfortably.  I
think you just need to be careful how you layout the I/O and you'll be fine
assuming you want some sort of normal performance metrics (normal to me is
no client-noticeable delays in action; delivery can be delyed a few
minutes). You may want to consider putting some of the IO in the server
itself by putting the queue/temp drives internally. The DL380 holds 6
drives, so you may consider putting queue/temp on a RAID 0+1 over 4
spindles, while the other two handle the OS/binaries/swap.  Then put the
Exchange db's and logs etc on the SAN along with the cluster parts that you
can run on the SAN.

Does that help?