macrumors.com has their hardware guide, but obviously sometimes Apple likes surprises even more and releases updates to hardware that according to the guide is "buy now - middle of life cycle", so you'll still have to read one or the other rumour site.
I guess there are a couple of guidelines that will help you when thinking about buying a particular (available) Mac model:
1.) When was it released in its current form? (macrumors.com's hardware guide is good for that.)
2.) Is an official Apple event near? (MWSF is early January, WWDC is in June, AppleExpo is in September usually. So if one of these dates are near, it's _never_ a good time to buy stuff, because a new version might be just around the corner.)
3.) Read rumour sites. You don't have to read them on a daily basis – and quite certainly not everything they say will come true. But if you want to buy an iBook and more than one rumour site says there's one around the corner, it might be worth looking more closely at all three points in this list again!
Plus, although you've already mentioned it and we _are_ here already: macosx.com. Create a thread like "Should I buy that iBook G4 12" right now?" along with information about _why_ you want to or have to buy it now. You'll certainly get a lot of info that _won't_ really help you in your decision (like: If you're into gaming, an iBook is not the solution, although for you it may be clear already and you aren't into playing computer games, anyway...), but you'll also get valuable opinions of people who have more or less experience in buying hardware at all the wrong and right times.
However: If you buy, then Apple updates and you do all these steps _afterwards_ (i.e. suddenly you find out that it all was so obvious etc.), you'll certainly get a few calls of pity but a lot of answers that are not _so_ nice...