Configuration Recommendations

I'm a n00b [1] to tuning DBs so if anyone has a bit of time to provide
feedback, I'd sure appreciate any input the community might have on the
plan, configuration, etc. I could very well have unintentionally left-off
crucial parts of my descriptions below and for that I apologize for wasting
time - let me know what I missed and I'll do my best to dig it up.

We are planning to rebuild our production 50GB PG 9.0 database serving our
application platform on the new hardware below. The web-applications are
80/20 read/write and the data gateways are even mix 50/50 read/write; one
of the gateways nightly exports & imports ~20% of our data. All
applications use a single DB but the applications themselves run on 6
different machines.

The new hardware for the 50GB PG 9.0 machine is:
* 24 cores across 2 sockets
* 64 GB RAM
* 10 x 15k SAS drives on SAN
* 1 x 15k SAS drive local
* CentOS 6.2 (2.6.32 kernel)

We are considering the following drive allocations:

* 4 x 15k SAS drives, XFS, RAID 10 on SAN for PG data
* 4 x 15k SAS drives, XFS, RAID 10 on SAN for PG indexes
* 2 x 15k SAS drives, XFS, RAID 1 on SAN for PG xlog
* 1 x 15k SAS drive, XFS, on local storage for OS

OS:
PAGE_SIZE = 4096
_PHYS_PAGES = 12,352,666 (commas added for clarity)
kernel.shmall = 4,294,967,296 (commas added for clarity)
kernel.shmax = 68,719,476,736 (commas added for clarity)
kernel.sem = 250 32000 32 128
vm.swappiness = 0
dirty_ratio = 10
dirty_background_ratio = 5

To validate the configuration, I plan to use memtest86+, dd, bonnie++, and
bonnie++ ZCAV.

If there are "obviously correct" choices in PG configuration, this would be
tremendously helpful information to me. I'm planning on using pgbench to
test the configuration options.

Thoughts?

Cheers,

Jan

[1] I'm applying what I learned from PostgreSQL 9.0 High Performance by
Gregory Smith, along with numerous web sites and list postings.