The hardware is of course a big issue, but wha should be more of an issue to you underlying this is the perating system. I moved to mac just over a year ago now, and i still run windows XP on a desktop, but buying a 12" powerbook is one of the best things i ever did. I have 768MB of ram, a 1.33 gHz processor, and a 60GB hdd. i use photoshop very often, and only ever reboot when a software update requires it. i can go to any website i like without worries of dialers, viruses or trojans... put simply it's a dream comes true. Right now my windows computer is broken (again) and once more the little powerbook has come to the rescue and i'm looking for fixes!
The laptop comes with some RAM in it, as i'm sure you know. when i bought mine, it was a 256 stick as standard, and with another 512 costing £50 or 1Gb costing £350, it was a bit of a no-brainer... however, i don't know if the Rev.D powerbooks ship with a single 512 stick or two 256s... might be something for you to check out there, as the SO-DIMMs that these things use aren't cheap.
The powerbook screen is very good, i'm used to the size now, but i'd hesistate to use it if it were my only computer. The processor never misses a beat, there's ocassionally problems when i've tens of applications open, but under normal - heavy (rather than stupidly heavy) usage, it's fine. Battery life, even after a year of use is still around the 3.5-4 hour mark using wireless networking, it goes up even more if i turn that off, but then no internet!
Get the mac. stuff just works, no messing, no frustration. it's the way it should be.
The mail should all transfer okay into Mail 2.0 on the mac, a very good piece of software which i use for managing 5 email accounts, a mix of imap, pop and http, which all sit together with separate inboxes. searching through 13 thousand emails using spotlight takes less than a second.
As for switching, it shouldn't take more than 20 minutes to find your way around most things, there really isn't a lot to it to be honest.
I know my opinion is biased, but i hope you make a choice which makes you happy.