Interesting! Both my work (home office) computer and HTPC are on 24x7. I also have a QNAP file server which runs 24x7 (it has a much slower processor than my PC's). If JRiver is installed on the file server, will the processor speed/memory matter? Meaning if I play an HD video file on my HTPC, which device does the video processing (file server or device it's played on?)
Some parts of video processing are always local (decoding and rendering), but, depending on your settings, other parts of video processing are done on the server. If transcoding is necessary, it has to be done on the server. It's better to run the server on a full-spec computer rather than a QNAP (if that's an option) for that reason alone, and it sounds like it's an option for you.
And what's the first step in setting up a system like that?
Open the Tools-->Options menu on the machine you want to use as the server. Go to media network and enable it. You should get an access key. Go to the proposed client, and under File-->Library select "Connect to a Remote Library." Enter the access key, and (if there are no firewall issues, etc.) the client should now have the same library, views etc. There are some tips and tricks after that, but that's three-click solution for getting started. Plenty more on the wiki and in the configuration options.
PS - I also have Plex running on my HTPC. I mainly use it to watch movies when I'm at my girlfriend's home using my phone and chromecast to her TV. I may be digging way in advance, but maybe JRiver can replace that in the future?
For the most part, yes, you just need to forward a port into the server on your router and you can then access the jriver server from anywhere just like plex using the access key. There are also jriver android apps that work as both remote players of media from the server -and- as remote controls for local JRiver instances. The only thing to note is that the android apps always require transcoding for video played to the android device (which is CPU intensive for the server), whereas full-fledged desktop jriver instances can take the video files "as is" with correct client-side configuration (which uses no CPU to speak of), so you'll need a lot of CPU power on the server if you have more than one or two people using the android apps for video at one time (about the same as plex for the same reasons).
The only hitch in your described use case is likely to be the chromecast. JRiver's apps definitely can cast audio to a chromecast, but I'm not sure if video is castable; my recollection is that there are workarounds via other unofficial android apps? You could also just plug in a laptop with JRiver on it in a pinch. I don't use chromecasts at all so can't offer any personal advice, but if you get to that point and hit a wall, start a thread and folks will chime in if they have any experience. I personally have a little intel compute stick that I use as a portable jriver instance, which, while not as cheap as a chromecast is much handier as it's also a general purpose computer