quad or dual core Intel CPUs

I am about to pull the trigger on a new machine after analyzing some
discussions I posted here last year.  I've been trying to spec out a reliable
and powerfull enough machine where I won't have to replace it for some time.
Currently I've been using a dual Xeon 3.06ghz with 4GB of ram and utilizing a
RAID 1+0 configuration over a total 6 SCSI disks asside from the OS partition.
We have about 10GB of data and will probably scale at about 1GB per month. We
currently average about 200 queries per second and the 15 minute load average
is about .30.  I am running FreeBSD 6.1.

At the end of last year, I specced out a new machine to replace this box. At
that time, the quad core 2.66ghz were not available from my vendor and I was
not planning to go that route. Now that they are available, I am considering
the option. The main question here is whether FreeBSD 6.X and PostgreSQL 8.1
would be able to take advantage of the quad core and perform better than the
3.0Ghz dual core.  The reason I ask is due to some conflicting benchmarking
results I see posted on the spec.org website.

Here is the full specification of the new box I hope to build and run FreeBSD
6.X and PostgreSQL on:

- SuperMicro Dual Xeon X7DBE+ motherboard
  + 2 x Quad Core X5355 2.66Ghz
    OR
  + 2 x Dual Core 5160 3.0Ghz

- 8 x 1GB PC2-4200 fully buffered DIMM

- LSI MegaRAID SAS 8408E w/BBU 256MB

- 16 x 73GB SAS disk

So, question #1, to go dual core or quad core?  Quad core in theory seems to
scale the machine's processing potential by almost a factor of two.

And lastly, up till now, I've only have experience configuring SCSI RAID
controllers.  I believe this LSI MegaRAID unit has a dual channel setup, but
when it comes to SAS drives, I don't know what kind of flexibility this
provides.  How should the disks be partitioned for maximum PostgreSQL
performance?

I'm thinking about keeping it simple assuming that the hot spare can only be
utilized one per channel leaving me only 14 disks to utilize.

1 RAID1 partition using 2 disks total for the OS
1 RAID1+0 using 12 disks total striping over 6.

If I am not able to utilize both channels to create a 12 disk RAID1+0 array,
then it might be better to create 2 seperate data partitions, one for
WAL/pg_xlog and the rest for the data store.

Please comment on any issues you may see with this box and my assumptions.
Also any FreeBSD kernel issues or tweaks you could recommend.

Sincerely,
Kenji