Well, I'm not an expert, but here's my take..
Are you going to take the hardware with you, or does that belong
to aD as well? Assuming you take it with you, I suggest:
Use the current hardware solely for Oracle. If I would be paying
$30/CPU Mhz or whatever the current oracle price is I'd make
damn sure nothing unnecessary is running on the db box taking
cpu cycles away from oracle. This has the added bonus of
increased security, you can put an additional firewall between the
webserver(s) and the oracle box.
First priority would then be getting a front end box for the
webserver. If you don't wanna play around get for example a dual
cpu 1U box from some big vendor, say IBM (who apparently has
good linux support). If you are a little more adventorous, the dual
AMD systems are apparently very good, and certainly give better
bang for the buck than Intel stuff. See for examplehttp://www.anandtech.com/IT/showdoc.html?i=1514
for a review of
one such computer (apparently very good indeed).
As for choice of linux distro, this week both Red Hat 7.2 and Suse
7.3 were released, and both of them are surely solid for server
stuff. In addition, Suse sells something they call "Suse enterprise
linux 7", which is supposedly more tested and includes 1 year
support for the price of $600. This distro is certified for oracle 9i,
in case you need new db hardware as well. All of these distros
have some journaling fs as default (Red Hat ext3 and Suse reiserfs
I think), which is a good thing. But for oracle, be sure to check the
possibility of using raw i/o, which I think is in the 2.4.x kernels.
It might also be a good idea to have another server for images and
static pages. Plain apache or maybe even the new in-kernel
webserver in linux 2.4.x would do just fine. As for hard disks. For
the webserver it probably doesn't matter so much. But as most
servers have built-in SCSI you might as well get scsi disks. Two
disks mirrored with software raid (if hardware isn't available of
course!) would give some redundancy against drive failure. But for
the db definitively go for raid10. If the db isn't absolutely huge, the
price difference between raid10 and raid5 is small potatos, when
you take into account the price of the rest of the system.