I used one for quite a while and it was fine with a single 1080p stream or 2 720p streams.
Then my kids started to stream and can sometimes have 5 X 720p streams going which killed the HP.
I then moved Plex Server to my Core2Duo machine, and while a little better, it still choked sometimes.
I now run the server on my dev machine which is an i7 Sandybridge. Plex is also installed on the 256GB Vertex 4 SSD which has made the starting of movies instant. I can work on this machine while the family are watching stuff and smooth experience all round.
So, short version is how you intend to use it.
Note that depending on the client, transcoding may not be necessary.