Is it possible to sync specific folders/files from my iMac to my iBook over wifi, replacing corresponding items on my iBook?
You can use DropBox to seamlessly sync data files between your two computers. Strictly speaking the files have to be kept in a DropBox folder for this to work, but there are work arounds. You have to sign up for the DropBox service and you get 2GB with a free account.
There are services similar to DropBox including GoogleDrive, SkyDrive, SugarSync, just to name a few but how far back they support older systems I don't know. Many have websites too so you can sync to the site seamlessly from the newer computer and use the Web site to download/upload data from the older one.