Manusz...
I would give up talking here if I'm in your chair. You know why I'm saying this.
As we all know, DNS-323 and built in firmware is... Crap.
But, CPU of DNS-323 is far better then D-Link support - means: FAST.
Workaround:
Install some other media server (mediatomb) onto your DNS-323 as I did on my DNS-313. Check this page:
http://forum.dsmg600.info/index.php
On mediatomb page you can find step by step instructions how to transcode video on the fly in DNS-323 to MPEG2, so your DSM-510 can read it.
And, what's the best:
You don't break support by installing some other software in the box (I installed torrent app, http server, UPnP server, ftp secure server, better download manager...)... Some other guys installed Debian onto DNS-323.