I am very happy with the the Raspberry's ability to handle lots of tasks at once, such as downloading/unraring/repairing/renaming/moving all only taking a maximum of about 30-40% cpu at its peak.
The streaming itself is using smb and it uses hardly any cpu or ram (at most about 5% at any one time). I personally don't download anything in 1080p so I don't know how it would handle it, the most I go is about 4GB for a movie and that worked seamlessly. To transfer a file of 4GB takes about 20 minutes so Its not super speedy, from that I can presume that if the file is over 12GB for an hours content it wouldn't work so well.
SMB shares take very little to serve up. However if you're talking about running plex I'm not sure. would that be on the revue itself as a server edition or is it just the client app like it is on the ATV2?
Because as far as I was aware Plex is much better on faster computers as it likes to transcode files before/as it sends them.
In terms of step by step guide not really, its a mix and match of various googled things.
I started with setting up shh/vnc as mine isn't connected to a screen:
along with making it auto login.
and then its samba shares:
It helps to use hexxah's rpi-update tool. I had some issues with kernel panic under heavy usb/network usage. So you might want to look into that.
After that it was just adding sick beard etc and then leave it running.