Apple updates and update order

One thing I'm wondering while figuring how to deploy InstaDMG is this: Aside from dropping an OS on an intended system for imaging, noting which updates are required, and noting the order of the updates...is there an easier way to determine which updates must be applied and in what order they must be applied to say a Quad Core G5 tower still in use with some pretty old install media? I'm quite certain we've held onto install media for each of the different machine types we still use. I'd just like to find a smarter way of identifying which Apple updates would be needed for a specific install CD/DVD on a specific machine type (PowerMac, iMac, MacBook, MacBookPro, MacPro, etc.) It's very possible I'm missing something obvious. Please feel free to jog my memory. Thanks.