This is too easy!
# apt-get install minidlna
And then edit /etc/minidlna.conf. Look for the line:
(probably line 26)
And write there the directory of your choice. Then:
# /etc/init.d/minidlna force-reload
And it should work (if you have a dlna client, of course). In my case, I have a Samsung TV with a RJ45 interface, capable of discovering DLNA servers on the network.
You can also have different directories for Music, Movies and Pictures, I recommend to read the manual of minidlna.conf:
$ man minidlna.conf
I have to say that the performance of RPi so far is quite good, I am watching a video perfectly while compiling Quake3 (my next task).