I'm not sure I understand correctly what you want to do (especially the part about "referenced" and "unreferenced" files. AFAIK, Lightroom doesn't give you the option to hide your image files inside the catalog file, the way Aperture does. If you want to keep your images folder inside the same folder as your Lightroom catalog, that's fine -- but they're still separate. Actually they're separate in Aperture as well; they're just hidden inside a MacOS X "file package.")
Anyway, if all you want to do is move all your photos from your old computers to your new computer without losing any of your Lightroom adjustments, croppings, keywords, etc., this shouldn't be difficult, and shouldn't require you to transfer all your images to an external hard drive first.
Instead, connect your old computer (either one) to your new one, either over your home network or (if you don't have a home network) through a Firewire cable. If you're using Firewire, you can start up your old computer in Target Disk Mode, which makes it behave like an external hard drive (consult Help for how to do this.)
However you do it, the object is to make sure your new computer can see the Lightroom catalog file (the one with the .lrcat extension) on your old computer.
Once the computers are connected, launch Lightroom on the new computer but NOT on the old computer. From Lightroom on the new computer, under the File menu, choose "Import from catalog..."; in the dialog box that appears, navigate to select the .lrcat file on your OLD computer.
Once you've selected the catalog file, you'll get a second dialog box with a "File handling:" option. DON'T choose the 'Add new photos to catalog without moving' option; that would just leave all your image files on your old computer. Instead, choose the "Copy to..." option, and select a location on your new computer where you want to keep all your raw-image files.
Once you've selected your destination and any other options that apply, click the "Import" button. Then walk away from your computer for a loooooooong time... it will take a while to copy the catalog data plus move all those 8,000 raw files over the network to your chosen location.
When the process is finally done, though, you'll have a full set of your raw files and a fresh new Lightroom catalog on your new computer, with all your adjustments, croppings, keywords, etc. just as they were in your previous catalog.
Now disconnect the first old computer, connect the second old computer, and repeat the above process to import its raw files and catalog data.
I assume there aren't any duplications among the photos cataloged on your Macbook and your G5...? If Lightroom does find any duplicated images during the import process, it'll ask you how you want to resolve their settings. You can, for example, overwrite the older ones with the newer ones, or vice-versa, or import the new settings while saving a virtual copy of each image with its old settings.
It might look a bit intimidating the first time you try it, but don't worry -- just work your way through the settings. Remember, if you mess up your import, all your raw images and your original catalog settings are still on your old computer; if all else fails, you can just throw away the imported copies that are on your new computer and try the process again!
Good luck...
