1)I am running a A Macbook Pro 2ghz with 2GB ram, and a 80gb 5200rpm drive. I run it opened up next to a 22" monitor with a keyboard, and mouse.
2)Running CS2 at the moment, cs3 is a huge speed boost for the intels as you can see in the link below.
3)My Photoshop settings are mostly stock, I increased the history states to 40, scratch disks is set to internal HD, from what i've been told using an external usb2.0 HD will not help anything. You can get an expresscard that will use a SATA connection to an external HD, that would make an excellent scratch disk, even better... a pair of 10k rpm raptors

buy more ram first though as Photoshop will use the scratch when it runs out of ram. (upgrade to 4gb if you can!)
4) I use a Wacom tablet, a dual display setup (as described above),and a Microtek i800 scanner.
5) Buying CS3 and more ram will help the MBP the most, I have 2GB (max for my model) and I still run out, OS X needs to use memory and if you have any other apps open, they use up the rest pretty quick.
Heres a link for benchmarks on photoshop using different machine and configs, I ran mine in 1min 33 sec. :
http://www.retouchartists.com/pages/speedtest.html