I just completed my first HTPC. I wanted a somewhat unique case, so I ended up using the case from an Apple IIGS. In fact, it was the same computer that I first used when I was a kid. Here's a Flickr set of the finished project:
http://www.flickr.com/photos/26652170@N ... 1744029063
The specs are:
Motherboard: ZOTAC GF9300-D-E LGA 775 NVIDIA GeForce 9300 HDMI WiFi Mini ITX Intel Motherboard
CPU: Intel Pentium E5300 2.6GHz LGA 775 65W Dual-Core Processor
PSU: Pico-PSU 150
HDD: Fujitsu MHZ2160BH-G2 160GB 5400 RPM 2.5" SATA
Memory: 2GB DDR2 1600 (2x1GB) It's being severely underclocked with this board, but I had some extra lying around so didn't need to buy any RAM.
I haven't used it too much yet, since I just set it up. So far, I'm pretty happy with it. Since the case is pretty low profile, and the top of the case lid is vented, the CPU fan actually works pretty well as a case intake fan. Then I just added a single 80mm exhaust fan to the back. It's not completely silent, but it's not any louder than the AC adapter on my XBox 360 when that's running.