Of course you can connect a Newton with a Powerbook ... The dongle and a serial cable permits it, just like others have pointed out. I myself have an old Apple printer serial cable with the dongle, and that's what I 'm using to connect the Newton and the iMac... You can also connect wirelessly with a BT card but you will need some drivers that you can find in the net. I recomend you to visit this site:
http://www.splorp.com/newton/faq/ that is very good to the newbie to seek fast answers to just all the questions that you can think of.
About the software, I use NewtSync to do the syncronization between the two machines and NewTen to install packages of software to the Newton from the iMac, and it's all very easy to do. Some say that NewtSync is a litlle bit buggy, I wouldn't know because I'm using the Newton more and more as an independent machine, but I remember that my first experiences with it were good, but as I said I don't use it at the moment...
Hope I could help
iMac G5 1.8 Ghz + Newton MessagePad 2100 Mac OS X (10.4.7)
iMac G5 1.8 Ghz + Newton MessagePad 2100 Mac OS X (10.4.7)