As far as I can see, there are two options for encrypted NAS.
The first is to have an encrypted file container stored on the NAS which is mounted by the client machine.
The second is to configure a dedicated server that mounts and shares encrypted partitions.
The first option would be more secure, but I have no idea what performance impact you would have piping it all through your network.
The second option is quite a bit more convenient and I believe it would be quite a bit faster as well, assuming the servers processor is capable keeping up with your workload.
It has an advantage of lower resource usage on the client side, but is less secure in that anyone on your network could conceivably access the files anytime the server has your drives mounted.
Probably there are more options as well (which I would also be interested to hear)?
I am assuming there are corporate quality NAS systems with better encryption options on the market somewhere, but I doubt they would be really affordable :/