Amazon has the QNAP 453be for only $409 (+ tax if applicable) if purchased using the 20% off Amex promotion that has been going around. I know there has been a lot of random deals citing this coupon, which I normally find annoying, but for everyone who has been looking for a deal on this, I figured it would be worth sharing. If you are looking to save some money, the 451+ is also available for $353.
This is similar to the deal @ Google Express a week or so ago, but even better since it's faster shipping (generally) and you know exactly how much, if any, sales tax you will pay, as opposed to the GE lottery of wherever they buy it from. That deal sold out very quick, but Amazon seems to have ~20 left in stock right now.
I purchased one myself last night since I've been needing an upgraded QNAP for a while now and this is as good of a deal as any on the upgraded and recent 453be model. More info on it: