The DNS323 is okay, but a bit slow to build the database if you have a very large media collection.
To give you an idea, here are the times for my media collection of 32,000 FLAC files (about 2,500 albums).
ReadyNAS Duo full database rebuild = 120 minutes
ReadyNAS Duo reboot db refresh time = 60 minutes
D-Link DNS323 full database rebuild = 60 minutes
D-Link DNS323 reboot db refresh time = 47 minutes
Qnap TS-219P+ full database rebuild = 19 minutes
Qnap TS-219P+ reboot db refresh time = 6 minutes
Qnap TS-659+ full database rebuild = 17 minutes
Qnap TS-659+ reboot db refresh time = 5 minutes
In operation, any will be fine for streaming music (I've tested them on 24 bit 192 KHz FLAC files (4.7 GB for a single album) and they all work well (about 9.2Mb/s if it were a WAV file, but as it's requested in chunks - to fill the media player buffer - it'll be about that rate even though it's a FLAC file) . The Duo and D-Link are a little sluggish when changing between some containers (menu items) when using my complex custom trees, but it's certainly no show-stopper. I've never tried them on anything other than FLAC files (so I can't answer any video streaming questions).
For your information: The ReadyNAS Duo has a Sparc CPU, D-Link has a 500 MHz Marvell CPU, TS-219P+ has a 1600 MHz Marvell CPU and the TS-659 has a 1600 MNz Intel Atom CPU. Newer ReadyNAS devices (like the Duo Ultimate) will be really fast as they have 1800 MHz Intel Atom CPU's (but I've never tried one).
My thoughts are that the D-Link would be fine, and if you want something really quick, I'd recommend the TS-219P+ as an ideal NAS. The other useful thing is that the Qnap's come with Twonky 5.1.6 pre-installed and you can easily update them to Twonky 6.0.30 by installing the official Qnap package of 6.0.30 (very easy; it's done via the Qnap settings web interface). The additional benefit is that Qnap have worked hard to make Twonky work very well and also that it is fully supported by Qnap. If you intend streaming high definition video at some stage in the future, you might need to consider an Intel based NAS (maybe even more powerful than the Atom ones). That's something you'd have to research in more detail. If it's normal video, I would assume almost any NAS would cope as the rates will be relatively low (but as I have said above, I haven't checked for myself).
Hope that helps
PS I've just posted some info on installing Twonky 6.0.30 onto a D-Link (that's when I spotted your question).