The original poster was asking for advice concerning media servers, yet most of the comments here have been about players.
Personally, I think the need for a media server has largely gone away. Most NAS devices support either DLNA, NFS, SMB, etc., and any media player worth its salt can find sources on the local network and play them.
Yes, if you want to be able to transcode on the fly, then Plex makes sense, but how much transcoding is really required these days.
My current set up includes:
Dlink DNS-325 running firefly that my Roku soundbridges / iTunes clients use for music. Yes, it's old school, but I love the soundbridges.
In terms of media players, I use the D-Link boxee box. It's not super fast, but it can use nfs to stream movies from the DNS-325.
I would like to run XBMC on it instead, but I'd lose Netflix if I did that.