Hardware recommendations

I need to build a new high performance server to replace our current production database server.
The current server is a SuperMicro 1U with 2 RAID-1 containers (one for data, one for log, SAS - data is 600GB, Logs 144GB), 16GB of RAM, running 2 quad core processors (E5405 @ 2GHz), Adaptec 5405 Controller with BBU.  I am already having serious I/O bottlenecks with iostat -x showing extended periods where the disk subsystem on the data partition (the one with all the random i/o) at over 85% busy.  The system is running FreeBSD 7.2 amd64 and PostgreSQL 8.4.4 on amd64-portbld-freebsd7.2, compiled by GCC cc (GCC) 4.2.1 20070719  [FreeBSD], 64-bit.
Currently I have about 4GB of shared memory allocated to PostgreSQL.  Database is currently about 80GB, with about 60GB being in partitioned tables which get rotated nightly to purge old data (sort of like a circular buffer of historic data).

I was looking at one of the machines which Aberdeen has (the X438), and was planning  on something along the lines of 96GB RAM with 16 SAS drives (15K).  If I create a RAID 10 (stripe of mirrors), leaving 2 hot spares, should I still place the logs in a separate RAID-1 mirror, or can they be left on the same RAID-10 container?
On the processor front, are there advantages to going to X series processors as opposed to the E series (especially since I am I/O bound)?  Is anyone running this type of hardware, specially on FreeBSD?  Any opinions, especially concerning the Areca controllers which they use?

The new box would ideally be built with the latest released version of FreeBSD, PG 9.x.  Also, is anyone running the 8.x series of FreeBSD with PG 9 in a high throughput production environment?  I will be upgrading one of our test servers in one week to this same configuration to test out, but just wanted to make sure there aren't any caveats others have experienced, especially as it pertains with the autovacuum not launching worker processes which I have experienced.

Best regards,

Benjamin