Filesystems are an interesting topic that I do not really enough about.
However, the manpage for mke2fs holds lots of information regarding the
tweaking of filesystem for new filesystems.  Additionally, tune2fs holds
information how changing some of them at run-time.

With respect to inodes, which is essentially where all of the metadata
about the files is stored, your larger filesystem (500GB) will
inevitably have a different set of optimized inode settings than your
smaller filesystem.  Different versions of the kernel and filesystem
tools will involve different filesystem defaults, which is another
likely case between your fedora 2 and centos 5 boxes.

With the number of images that you proportedly have, you might want to
use the "dir_index" feature of ext3 to speed up directory lookups (make
sure you run fsck.ext3 after tune2fs to change it).  You could also use
the "super_sparce" feature to save some more space.  Again, all of this
info is in the manpages.

The last thing to consider regarding effective use of space.  You might
want to re-create the filesystem on the RAID 5 array to match the
"stripe-size" using the "stride" extended option (mke2fs).

For more information about ext3, here's a transcript of a presentation
by Dr. Stephen Tweedie:

http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html

Enjoy!

Chad