I run all my media from a single PC with a large hard drive.
I use the Media Server for streaming to all of my clients (3 other PC's and a Tivo) and do all my library editing from the Media Server. This works the best for me and all I have to backup is a single machine.
My only major complaint, and this is not for JRiver, is that Tivo doesn't let me stream my home movies yet. I am hopeful that with the recent Tivo DVD burner
http://www.pioneerburner.com that streaming video is in the near future.
As far as HTPC advice, you will want to make hardware choices based on your ultimate goal. Is for digital media editing? Digital media playback? Games? A PVR? All of the above?
Video editing will need a lot a RAM. Games will require a really good video card and CPU. A PVR (tivo-like functionality) will require a big hard drive.
The more things that you will want to do with the machine, the more hardware you will require. All this comes at the cost of power consumption and heat dissipation. This means bigger power supply and noisy fans.
Ideally, you will want a good video card with DV output for a HDTV and a good sound card with 5.1 or digital audio outputs for integration with your receiver.
Ideally, your Media Server should use a SCSI drive array configured with RAID5 for maximum security and reliability. But this can be expensive, you could do an IDE drive array for less money (and performance).
There is a decent article about this on the Extreme Tech website
http://www.extremetech.com/article2/0,3973,488627,00.asp that might be worth a read for someone starting out a new project. At least it points out some of the things to think about.
-Shadowen