If you put your photos on an NAS, you can use the former external drives for a backup of the NAS, I've done that for years.... If you've been keeping anything other than scratch work on your desktop, you are brave, I don't trust them with all the viruses on the web... Unless it has a dedicated boot/software drive and a separate drive for data.
Last.. NAS, What do you want to spend? If you use JPEGs, it shouldn't matter much, they are usually small, all RAW files and you need a bigger faster NAS. You can easily get speeds similar to what you have on your local hard drive, but it costs money. I'd say for that speed, at least a couple of grand. Close to $2000 for the NAS and then you need the drives to populate it. A $200 NAS with drives built in, is going to be slow. Of course you should try the one you have now and see if its fast enough.
Too Much Camera Stuff......