Question on Opteron performance

Right now, our production DB server is getting a bit more heavily loaded
than we'd like, and we expect its usage to double in the next few months,
so we're looking at where to put our money for a better machine.

Right now, we're using a dual 2.8GHz Xeon with 3 gigs of memory, and run
without fsync() enabled. Between disk cache and shared buffers, the disk
system isn't an issue - vmstat shows that the disk I/O is nearly always at
zero, with the occasional blips of activity rarely being more than a few
hundred kilobytes.

The main question in my mind is whether a 4-way Opteron is going to
give me enough of a performance benefit over a 2-way Opteron to make the
extra $10k worth it. My first guess was that it would, as going from 2
Opterons to 4 will give you twice the potential memory bandwidth.
However, as PostgreSQL pulls heavily from the global buffers, I may not be
able to utilize all of that potential bandwidth.

If anyone has done tests with PostgreSQL on 2- vs. 4-way machines under
heavy load (many simultaneous connections), I would greatly appreciate
hearing about the results.

Steve Wolfe