Right now OS X will limit any one application to something like
3-4GB. I am sure someone can step in and give the exact number.
Not sure if Leopard will fix that or not...but unless the OS lets
it happen, a 64-bit app on its own is not going to be able to use
more than what the OS allows.
That said, if you are running multiple apps, like Lightroom and
Photoshop, or Aperture and Photoshop, plus a few other things...you
can easily start to chew up a lot of RAM quickly. So, getting 8GB
or more of RAM will not be a waste IF your workflow can use it.
Jeff
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http://www.pbase.com/jhapeman