I usually wait for installing a new version of the Mac OS. But if you have what you wish to keep backed up, then you should get along all right. I still just find it simpler to wait. But I would not wait long for a new Mac. Panther is quite good.
It sounds odd, the idea of having to install the original OS, then the newer one. Why?
I got a G5, and am quite happy with it. It is supposed to be that come January Apple will have a PCI express graphic slot, with PCI X slots. We shall see. New motherboard for that is needed.
It seems like it would be a while before everything is 64 bit. But people may well be more knowledgeable than me. So many developers/supplier-vendors have 32 bit programmes, and they will have to run side by side with the newer programmes from developers. It is hard to imagine Adobe and Quark being quick about getting 64 bit support in good fashion, when so many of their customers will have 32 bit OSs.
May you find much enjoyment and fertile use of your new Mac.