The main difference between the iBooks and Powerbooks is the Powerbooks will drive a second monitor for increasing your desktop space. I use an old 22" CRT for my main monitor and the my Powerbook's screen for my second monitor for pallets and stuff when I'm home working in my office.
The iBooks only support video mirroring so if you plugin an external monitor it will show the same thing as the build in screen.
Also the 15" and 17" Powerbooks have a PC card slot that most people never need. I do use mine for reading my camera's flash card and sometimes to gain more FireWire ports but most people don't need this.
The Powerbooks are also somewhat faster, have lighted keyboards, etc.
Here is a list of software that you will find on a Mac's drive.
http://www.apple.com/ibook/software.html
I see they are still listing Appleworks which has been replaced by iWork so I'm not sure which they are giving. Appleworks is not very good.
For commercial software here is a small list
http://www.apple.com/macosx/applications/
You can run most nix apps in X10 like OpenOffice, Gimp, etc. As long as you don't mind giving up the Mac interface, drag and drop, things like that they are fine.
The iBooks have some RAM soldered on the mother board and one slot. Macs love RAM so get as much as you can, it really helps.
All the current Macs are fast so it isn't really an issue unless you are doing video work or high end Photoshop. I do video work, Photoshop work, and all my web work on my 17" 1Ghz. Powerbook at it is nice as fast but not as fast as my neighbor’s duel processor 2Ghz. G5 tower that he has.
List of retail Apple stores.
http://www.apple.com/retail/
ibook page
http://www.apple.com/ibook/
Powerbook page
http://www.apple.com/powerbook/
I usually buy my stuff mail order either from MacMall or ClubMac.
http://www.macmall.com/
http://www.clubmac.com/