In my experience the worst things about using a PC for media playback are:
1. The fan noise.
2. The optical drive noise.
3. The hard drive noise.
and way down the list
4. The quality of the internal soundcard.
1. Build a fanless computer. It's possible to get enough processing power to decode 1080p video with 7.1 lossless audio, that does not require any active cooling.
2. The easiest solution is to only use the optical drive for initial ripping, and play back from the hard drive instead. But in some cases (e.g. Blu-ray discs), it is difficult to rip and preserve enough quality. So secondly make sure the drive has a quiet mode for streaming playback.
3. The ideal solution solid state drives. But that can get expensive when backing up movies. So make sure the drives are quiet. Or you can locate them in another room, and access the larger stuff via network, with a small SSD for the OS.
4. I've never splurged on a "hi-fi" internal soundcard. By the time I started messing with the idea of having a small studio, just jumped to a card with an external interface. The problems with the internal cards, is they almost always have tiny 3.5mm phone jack. I find those to be noisy, or to get noisy if left to oxidize (the contact point is just too small). Also a lot of the internal stuff isn't shielded very well, but I see the hi-end cards do offer that protection. My solution been to get the bits out of the computer in digital form, and turn them to analog with a dedicated device. The easiest way to do this with a modern setup is via HDMI, and you're done. If you're mostly interested in audio, which you've said you're not, then an audio interface connected by USB or Firewire, with XLR jacks is ideal (and not that expensive).
So to reiterate: Build a quiet PC, and connect it via HDMI to a receiver. Or you can get an external audio interface with at least 8 output channels and connect them directly to power amps. Add a network server as an option.
PCs can't be bad for audio, because every CD/DVD/BD released these days is mixed and mastered on a PC (or Mac).
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs