Choosing the Hardware
This build focuses on building a budget Media Center PC, tuner excluded, for less than $500 (including OS). Tuner's vary in price depending on need, but it was a goal to have a dual tuner OTA/ClearQAM DVR for less than $600 when finished.
Case - nMEDIA PC HTPC 1080P
Like all PC cases, you can spend next to nothing or a small fortune to house your components. We're going budget, so let's stick to the former. If tucked out of site, you can use traditional case form factors, like a mid or mini-tower, but they also make specific HTPC form factors to look more like video components.
For this build, I've chosen the HTPC 1080P from nMEDIA PC. It's $50, small form factor, and compatible with Micro-ATX boards.
CPU - Intel Pentium G620
Intel or AMD? Hmmm... it's always a tough question at the lower end. You can go either way, but to maximize power savings and performance at the budget end it's really difficult to look away from Intel's current crop of Sandy Bridge Pentiums, namely the G620 at $64.
The weak link here is the graphics. The Pentiums lack some of the advanced graphics features the Core i3's support, but maintain the H.264 acceleration and audio bitstreaming through HDMI (which is arguably all we really need anyway). AMD's Llano APUs would give it a huge kick in the graphics department (at the expense of CPU performance), but we could get the best of both worlds here with a low-power add-in graphics card.
Motherboard - ASRock H77M
You'd likely save a few bucks going with an H51 based micro ATX board, which will be fine in most setups, but the few extra dollars here buys you flexibility and some other nice features. When building HTPCs, I tend to build them to integrate into any setup. This board sports HDMI, DVI, and VGA, optical, and six channel analog out. You also get USB 3.0 and SATA 6, which could come in handy down the road.
RAM - 8GB G.Skill Value RAM
Technically, 4GB is sufficient for a Media Center PC right now, but with DDR3 so cheap, the price difference between 4GB and 8GB is negligible. For this build, I'm going with the G.Skill Value 2 x 4GB kit for $40, but you can go with whatever suits you.
Hard Drive - Western Digital Caviar Green 1TB
Now that hard drive prices are starting to become a little more reasonable, it makes sense to pick up a drive capable of storing a boatload of TV shows. Western Digital's 1TB Caviar Green drive does the trick. It's 1TB size lets you record a lot of content, the $95 price tag doesn't kill the budget, and it keeps power consumption minimized.
Cooler - Stock
The stock Intel Cooler for the Pentium is sufficient for a Media Center PC and pretty quiet.