The one thing I would strongly recommend for image processing is lots and lots of RAM. Fast processors and fast hard drives are nice, but the machine will grind to a halt once you have multiple layers and whatnot filling the RAM and the machine starts swapping to disk. Consider 1GB of RAM a minimum. Get 2 or more GB of RAM if the machine will support it.
I too would recommend an Apple product.
If you have a real need to be portable, consider a laptop. Otherwise get a desktop. They are generally more powerful and don't obsolesce as fast. They are almost always easier and cheaper to upgrade.
You didn't mention a budget. What are you willing to spend for a) hardware and b) software?
-Chip
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