Why should I buy another ReadyNAS?

You'll also get the benefit of being able to back up one NAS straight to the other with Replicate. I've never used it, or looked into it's features, but If the marketing blurb is to be believed, it should be a doddle to back up your new NAS to your old one and use the old one as a backup store. Obviously, if you get an NV+ and fully populate it, you'd get to a point (perhaps) where you have more data than the the backup NAS can store, but that is further on down the line.

Also, having two systems by two different manufacturers means you are duplicating your requirements for support. The HP N40L looks like a nice piece of kit, but will it be supported by HP for the length of time the ReadyNAS line has been supported by Netgear? I have my doubts.

If I were in your shoes, i'd go with the ReadyNAS NV+ V2.

[EDIT]: looks like Replicate is x86/ARM only from the add-ons page. However backing up from one ReadyNAS to another should still be a lot simpler than with two different products by different manufacturers. I'd still go with the NV+ V2 considering they are both the same price from the support viewpoint.