I agree with most of the responses, but I didn't notice anyone address the laptop hard drive (HD) itself other than about size. One of the biggest bottlenecks in a laptop is it's internal HD speed. Generally, laptops ship with 5400 RPM drives, unless you get something more geared toward high end gaming.
Even if you're storing your pictures on an external HD, the internal drive still runs the OS and Photoshop. Your best bet is to find or even request that a 7200 RPM HD is used. I agree with ELAY about the RAM, 2 GB should be plenty. As for the OS, Dell still offers laptops w/ XP (
http://www.dell.com/content/topics/s...=19&l=en&s=dhs). If you decide to go with Microsoft over Mac that is.
Cheers,
Chris G.