Take a look at the features on the new units - my DNS-323, which I've owned since they were released in 2006, failed earlier this year - newer units come with more processing power, more RAM, more flash and that gives you the ability to do more.
Even though my DNS-323 did pretty much everything I asked of it (sit in a corner and backup the critical data on my SOHO server), it was missing a couple of essential features, like the ability to shut itself down under UPS command (yes, I know it was added, but it never worked properly), and the ability to restart itself when power was restored.
Take a look around and then make your choice.