Configure redundant NAS boxes

I run a network of about 20 machines that all are located at the same site. I am using Microsoft Server 2003 Small Business Edition for Active Directroy and Exchange, but nearly all of my file serving is handled by a NAS box. I am using a NETGEAR ReadyNAS NV box.

I am backing up the data stored on the NAS box to external hard drives daily. However, I would like to have a backup NAS box that is immediately ready to go in the event of a failure on my primary NAS. I've purchased a second ReadyNAS NV and I need some help on the best way to configure this.

My first instinct would be to put the backup NAS on the network and run nightly backup routines to sync the primary and backup NAS boxes. This sounds fine to me, but I'm wondering if there is a better solution. Does anyone know if there's a way to configure these NAS boxes so that they replicate automatically?

To summarize, I need
1) A backup NAS that is ready to go with almost zero downtime in the event of a failure on my primary NAS.
2) The backup must be synced with the primary at least daily.
3) A small impact on performance on the primary NAS.
4) The backup must be invisible to users until such time that it is activated as the primary.

Thanks for any help.