belatedly, not having logged in for a couple of weeks, I'm using an Apple iBook as my number 2 machine, and I'd say go for it - with one caveat: upgrade the RAM in the iBook as a priority. Apple is really miserly about RAM.
But apart from that, it's great: I run my PC and the Mac on my wireless network painlessly, sharing files and the bb connection. I also discovered quite by accident recently that if you have your music collection running under iTunes on the main PC and you've got iTunes open on both the PC and the Mac, the Mac will automatically see your music and offer to play it! Very cool!