I have NO problem at all running 2 movies (LotR 1 & LotR 2, action sequences) 1080p at the same time from my DNS-323 using D-Link's NFS & having Jumbo frames enabled.
This is how I've mounted it using /etc/fstab
192.168.0.2:/mnt/HD_b2/Media/Movies /home/user/Videos nfs _netdev,auto 0 0
I'm having a D-Link DIR-655 router as well.
Runs perfectly fine. I'm actually considering buying a 2nd DNS-323, or maybe even a DNS-343! Great joy for that price.