Mmh, I have USB 3.0 ports on my newer computer but no devices that use it. Wikipedia says that you can plug USB 3.0 devices into USB 2.0 ports. It will then, of course, be just as slow and not offer the additional stuff USB 3.0 does. It's possible that USB 2.0 devices are cheaper.
You can use external HDDs as storage devices and copy your files there. I don't know how much sense employing a ghosting program makes in your case. It depends on whether you end up buying a new machine after the HDD failure or not and whether you want to have a completely fresh install of the OS and programs you use. I'd prefer the latter so I'd just save files I want to keep.
I don't know how Norton Ghost does things but usually, these programs make incremental updates to the back-up, i.e. they add new files to the existing back-up. I use a back-up program that came with my OS to back-up my home directory (I'm on Linux where things work a little differently) and the back-up is just a directory on one of my external HDDs. I also have my music collection and other stuff on that drive. Of course, depending on what you decide to back-up you might not have the space to put anything more than the back-up on your external HDD.