which dual-CPU hardware/OS is fastest for PostgreSQL?

I'm sorry if there's a URL out there answering this, but I couldn't find it.

For those of us that need the best performance possible out of a
dedicated dual-CPU PostgreSQL server, what is recommended?

AMD64/Opteron or i386/Xeon?

Linux or FreeBSD or _?_

I'm assuming hardware RAID 10 on 15k SCSI drives is fastest disk performance.

Any hardware-comparison benchmarks out there showing the results for
different PostgreSQL setups?

Thanks!