Also known as a "home theater PC" (HTPC), a media center PC is connected to a TV and is built and configured for streaming / downloading media available on the Internet or local network.  I personally label it a mere hobby, but I admit that building the ultimate media center PC has turned into my obsession.  I'm like Goldilocks seeking the one that's "just right."  So much of my time, money, and effort have been spent researching, testing, building, and configuring that I thought I'd share my discoveries hoping to spare others from the inevitable torture that accompanies this project.

These were my requirements, which may or may not be in accord with yours:

• The system must have a DVD player.
• The system must have an HDMI output--one cable running to the TV.
• The system must support a resolution of 1920x1080--"Full HD."
• The system must be small form factor and quiet.  I don't want a fat-ass noisy desktop connected to my TV.
• I don't need TV recording features (e.g. a TV tuner card).
• I'll try to get it as appliance-based as possible.  In other words, I will try my damnest to get it running on a minimum Linux install.

This was my mission, and here is my story.

The Hardware
The most formidable step is the first one--choosing the right hardware.  I tried a veritable heap of hardware, so instead of boring you with droning dialogue, here's the summary:

Apple Mac Mini
I don't think the earlier-gen Mini was fully intended to be a media center PC, and considering that it functioned decently.  But my eagerness to tackle the challenge of Linux plus desire for a better remote and HDMI output ended my honeymoon with the Mini.  Looking at Apple's website, the new Mini inspires a more focused vision for a media PC; however, that price tag pushes me to file that idea under "Expensive Plan B."

Positive

• Easy to get working.
• Software like Remote Buddy makes remote control configuration simple and versatile.
• Very small form factor and quiet.

Negative

• The Apple remote kinda sucks.  I hate the minimalist approach and the buttons don't respond well.  Apple has since redesigned the remote, so maybe this has been remedied.
• No HDMI out.  This has since been remedied in the next-gen Mini.
• RAM is a pain in the ass to upgrade.  Again, this has been remedied in the next-gen Mini.
• Power button is on the back.  Ugggg...
• Video would sometimes not come up upon boot.  This might have been due to the DVI to HDMI converter I had to use.
• Pricey.  It's Apple.  What do you expect?

Dell Studio Hybrid
The Hybrid is an attractive little piece of hardware, but not exactly right for my media center PC.  The intermittent HDMI video issues coupled with the frustrating lack of a working DVD eject button demoted this box to a "just in case" Windows PC.

Positive

• Stylish look.
• Runs quiet and cool.
• HDMI out.
• Super easy to open up and work on.

Negative

• Stupid touch-sensitive DVD drive eject button requires a special driver to work.  You think Dell makes that driver for Linux?
• Video would sometimes not come up upon boot even with direct HDMI out.  I think this is a common problem with first-gen HDMI-out motherboards.
• Pricey, especially for a Dell.
• Integrated Intel 945 video has limitations.

Dell Inspiron 400 Zino HD
Maybe I was just unlucky enough to get a lemon, but this machine was a nightmare.  Having opened it countless times, I concluded that the build and design quality is not nearly as good as the Hybrid.  Who sticks a full-size desktop hard drive in a mini form factor, cools it with one small fan on the back, and expects it to work?  Dell!  Needless to say, I'm not a fan of Dell's stuff anymore.  Hopefully the next gen will be better because this one is a bright idea cursed by poor design.
If you do buy one, I recommend buying extra SATA cables and switching the full-size HDD with a 2.5" one.  Oh, and the SATA power is a funky 5-pin mini connector that neither I nor the Bic Camera clerk had ever seen before.  Expect to bust-out the soldering iron to MacGuyver those new SATA cables together.

Positive

• Impressive specs.  The ATI Radeon HD looks incredible.
• HDMI output that's actually reliable.
  
• Quiet.
• DVD drive has an eject button!

Negative

• Runs hotter than hell.  This thing gets so hot it's a miracle the plastic outer body doesn't melt like ice cream.
• Not surprisingly, it started having I/O issues.  At first I thought the hard drive was overheating, but weeks of troubleshooting later, I figured out the SATA cables were going bad.  I guess SATA cables don't last long when baked like cookies.  It's working fine with new cables, but for how long?
• The 3.5" internal drive is laid flat across the CPU and GPU like a fish on a grill.

Home-built PC
What an injustice it is to buy a name-brand PC and then have to MacGuyer the hell out of it!  Therefore, I did something I hadn't done in many years--built my own PC from scratch.  This was the "just right" porridge that me and Goldilocks had been hunting for (she's really hot now, by the way).  Here's what I used:

• ASRock A330ION motherboard
• AOpen S110 small form factor chassis
• Pioneer slot-loading slim DVD drive (w/ eject button!)
• Transcend 32GB SSD
• Transcend DDR3 1066 RAM (4 GB)

So far so good on this one.  The HDMI video comes up on each and every boot, and that says a lot considering my previous experience with the no-video-on-boot-up curse.  It does run a little hot, so I added a fan to the GPU.  I went with SSD thinking it would keep the system cooler, but that probably wasn't necessary.  Also, 2 GB of RAM probably would've been sufficient.

Positive

• Reliable HDMI output with my TV
• NVIDIA ION video looks clean and clear
• Super quiet
• I got exactly what I wanted (like a slot-loading DVD drive with eject button).
• I can easily modify / upgrade it later.  New motherboards targeting the media center niche are coming out all the time now.

Negative

• Runs surprisingly hot for an Intel Atom-based machine.
  
• Took more time to get up and running since I had to build it myself.
• Won't be easy to resell (on Ebay, Yahoo Auctions, etc.) if I ever want to.

Learnings
My constant hardware tinkering taught me a few key points regarding media center PCs:

• Systems with NVIDIA or ATI video just seemed to work better than Intel video.  I didn't have much luck with Intel video--particularly when using Linux.  For example, the Intel video on the Dell Hybrid required manual configuration to support 1080; ATI and NVIDIA worked perfectly after installing the restricted drivers.  Going forward I'm sticking with NVIDIA and ATI.
• Getting a motherboard with a reliable HDMI output for TV use was an unexpectedly daunting challenge.  It's improving as hardware generations progress, but getting over the no-video-on-boot-up issue was a frustrating and expensive experiment.
• Cooling is of paramount importance with these boxes because they're so small and often placed in poorly-ventilated media cabinets.  They're also expected to run whisper-quiet so as not to interfere with our viewing experience.  Machines built with laptop parts (Mac Mini, Dell Studio Hybrid) ran significantly cooler than those built with desktop parts (Dell Zino).  Be wary of small form factor PCs built with desktop parts.
  
• Hardware is a pain in the ass to shop for because you never know how it'll work with your TV until you've bought it and connected it.  By then you're married to it.  Seemingly insignificant things like DVD drive eject buttons and the number of USB ports made a significant impact on my user experience.

Other Ideas
Here are other manufacturers I investigated.  I've heard and read positive things about their hardware, so I list them here as potential starting points for those diving into the world of media center PCs.

• ASRock - The maker of my motherboard of choice also offers small form-factor media center PCs.  OEM'd as "Valore" here in Japan, I was seriously considering it before deciding to build my own machine.
• AOpen - The maker of my chassis of choice also offers bare-bone HTPC kits.
• ASUS - Their evolving EEE Box series has impressive media-centric specs.  Their latest model even has an optical drive.
• Acer - The Aspire Revo series has an almost cult-like following as an inexpensive media center PC.  Lacking an optical drive, however, I crossed it off my list.
• Zotac - Like Acer, Zotac fills the HTPC niche market with their well-received machines.