Digital BW, The Print

Very few apps can utilize 8 processors simultaneously, and they are
largely video editinging/production tools.
See his blog post by John Nack for more info
<http://blogs.adobe.com/jnack/2006/12/photoshop_and_multicore.html>
Even if PS doesn't utilize all available processors that doens't mean
there's no benefit to say, a 2x dual core setup. You have "spare"
processors to devote to other apps and the OS while PS is crunching
away. But video pros aside, the 8 core machines seem to be more for
geek status than real world performance gains.
When I switched to CS3 I moved to a Mac Pro 2x 2.66 dual core from a
dual G5 2.5. On the Mac Pro, PS launches faster and progress bars feel
shorter. A noticeable difference? Yes, but not night and day. And of
course for optimum performance you need to feed the beast with
adequate RAM, use fast drives, create RAID scratch disks, etc etc. So
the cost of upgrading goes far beyond the Mac Pro purchase. Oh, and on
top of that you get to track down (and pay) for upgrades for your
other heavy use apps that must now be Universal Binary to run at full
speed.
Having said all of that, I don't regret the move, and everything feels
fast, but there are definitely other ways to spend your money that are
more photographically fulfilling.
-- 
amadou diallo
Author, Mastering Digital Black and White
www.masteringdigitalbwbook.com