LM,
While I have yet to actually install and run JRiver, your question is more of a general network question that would be independent of the software you use.
If you have the money and space, I always recommend building a decent server to store all your media. While NAS's are now coming with Gigabit networking, iTunes, Twonky, DLNA, etc. built in there are several disadvantages: expandability; ability to install any media-serving software, speed, are probably the top three. The biggest 'Pro' for a NAS is power savings, second is usually space.
To get a really good NAS, though, can sometimes cost as much as putting together a good server. As your usage of media grows it can easily be expanded. To save costs, you could run linux. Windows may give you a bit more flexibility due to the quantity of media software available on that platform, especially if you want to eventually install tuner cards in the server. Why install a tuner card in a server?
JRiver (and some other vendors out there, but JRiver seems to be the best solution so far) makes the tuners available on the network, so other computers/DLNA devices can actually control them. It's really a slick setup and what I will be doing once I finally piece together my next server and get JRiver installed (currently working on the wife to get her prepped for a change from the current status quo).
I would definitely set up a central "server" though. Remember, it doesn't have to be too crazy. Any computer with a shared folder is a server. Having a central repository really makes it simple for sharing (but it is also a single point of failure so don't forget backups). Maybe start off simple. An older computer to test it out. If you like it and eventually upgrade, keep the computer name and share-folder-names the same and none of the computers on the network will know the difference.
Put music, photos, and movies in different folders. How you set those directories up is up to you.
Best of luck.
hEdly