As long as you aren't expecting to run some customized business software or have the same selection of games, an iBook should work great. VirtualPC can help in some cases if you get stuck, but it isn't the fastest thing around. And if you do any online banking you should check to see what web browsers it supports as not all will support Apple's Safari web browser. Most will support Mozilla/FireFox/Netscape though.
Mac OSX is very hungry for memory, so I would recommend upgrading as far as you can afford to. 512MB is the minimum to have it run smoothly, but 1GB may be preferable if you like to run several apps at once.
There are a couple of sites with articles about switching to Macs (can't remember offhand) that might help. I think Apple has something for "Switchers" actually.
My brother loves his iBook and my parents just bought a Mac Mini. Very nice computers to use.
goates