I have found a great media server that works and is easy to use (with a nice GUI)! It is made to work with the PS3, but works great with the DSM-750. It also transcodes the video. I will be trying this out on the DSM-520 later and report back. I am curious though if it will have trouble with the larger video files over 2 gigs as the D-Link Media Server does. Ill check that out also and report back.
http://code.google.com/p/ps3mediaserver/
Current features
    * Ready to launch and play. No codec packs to install. No folder configuration and pre-parsing or this kind of annoying thing. All your folders are directly browsed by the PS3, there's an automatic refresh also.
    * Real-time video transcoding of MKV/FLV/OGM/AVI, etc.
    * Direct streaming of DTS / DTS-HD core to the receiver
    * Remux H264/MPEG2 video and all audio tracks to AC3/DTS/LPCM in real time with tsMuxer when H264 is PS3/Level4.1 compliant
    * Full seeking support when transcoding
    * DVD ISOs images / VIDEO_TS Folder transcoder
    * OGG/FLAC/MPC/APE audio transcoding
    * Thumbnail generation for Videos
    * You can choose with a virtual folder system your audio/subtitle language on the PS3!
    * Simple streaming of formats PS3 natively supports: MP3/JPG/PNG/GIF/TIFF, all kind of videos (AVI, MP4, TS, M2TS, MPEG)
    * Display camera RAWs thumbnails (Canon / Nikon, etc.)
    * ZIP/RAR files as browsable folders
    * Support for pictures based feeds, such as Flickr and Picasaweb
    * Internet TV / Web Radio support with VLC, MEncoder or MPlayer
    * Podcasts audio/ Video feeds support
    * Basic Xbox360 support
    * FLAC 96kHz/24bits/5.1 support
    * Windows Only: DVR-MS remuxer and AviSynth alternative transcoder support 
This is the site where I found a large list of other DLNA Media Servers:
http://www.rbgrn.net/content/21-how-to-choose-dlna-media-server-windows-mac-os-x-or-linux