Most any computer forum will be helpful, but it's pretty simple if you have built a computer before. For the most part, when building an HTPC, you want:
Case - Most HTPC cases are smaller than standard ATX cases and are usually positioned on their side. Cases can range from $50 up to $300 with design and materials generally being the main differences. The more expensive ones will generally run cooler, be built better, have less annoyances (i.e. rattling), and will generally be easier to work w/.
Power supply - In an HTPC computer, you won't need much power. It's more important to have a quiet and efficient power supply, so choose one that's 300-400 watts and is 80-plus certified.
Motherboard - Depending on what you're using it for, you may want to go w/ motherboard w/ integrated graphics (i.e. AMD 690G or Nvidia 7050 PV) or, if you go w/ dedicated graphics, it won't really matter too much. The AMD 690G is probably the best choice overall, IMO, as it is a fairly efficient chipset, offers HDMI w/ audio, and is generally pretty reliable. If you're not too big into computers, Asus and Gigabyte are safe bets for motherboards.
CPU - CPU won't matter too much as long as it's fast enough. Power consumption and heat generation is more important and the AMD Athlon64 X2 BE-series is the best choice, IMO. It has a 45 watt TDP (thermal design power), making it the most efficient CPU available for the desktop and, while they are priced a fair amount higher than their 65 watt counterparts, I think it's worth the extra $20 to save 20 watts for an HTPC.
Memory - Realistically, 1 GB of memory would be fine for an HTPC that's doing SD content. 2 GB would be fine for HD. However, w/ the prices of 4 GB kits these days, why not spend the extra $30-$40 ? You won't be overclocking, so any 4 GB kit will work, really.
Hard Drive - Probably the most important part of an HTPC as it will store the information. HTPC cases are not large, so if you're going to store the drives in the HTPC itself, you'll want a high capacity drive. Like anything in an HTPC, having a quiet and power efficient component is also a plus. I chose the Western Digital Green Power 1 TB drive because it's huge, quiet, and power efficient and the cost isn't too bad, either. If you want lots of storage, you may want to look into NAS (network accessible storage) enlcosures for a couple reasons. The first is it allows you unlimited storage in a different location (say a closet, cabinet, etc.) and the second is it removes a source of noise and heat from the HTPC.
Graphics Card - For SD content or even HDTV, the onboard video utilized on a 690G or 7050 PV board will be more than sufficient. However, if you want to get into Bluray or HD-DVD, you'll probably want to look at a dedicated GPU as the integrated ones won't be able to cope. The AMD/ATI HD 2400/2600 series GPUs are the best choice at the moment w/ lower power consumption and slightly better HD decoding than Nvidia while offering audio over HDMI (something Nvidia doesn't have yet). If you're not a fan of ATI, the Nvidia 8400/8500/8600 GPUs would be their equivalents. There is NO point in getting a high-end GPU as they won't offer any better HD decoding while increasing power consumption and heat generation significantly.
Optical Drive(s) - The base HTPC will have a DVD burner, which will run in the area of $30. The main area to look for is noise output for the most part. Samsung and Pioneer are some of the better brands in this area. If you want to get into HD-DVD and/or Bluray, those drives can be had for about $200 (read-only).
TV Tuner(s) - I'm not a TV tuner buff, so I'm probably not the best person to ask on this topic, but tuners are available in a wide variety of options and qualities. Make sure you get one w/ a remote and that it is compatible w/ your OS.
Operating System - You'll want an operating system that features a media center application such as Windows Vista Home Premium/Ultimate or Windows XP Media Center Edition. There are also many distributions of Linux that are built around media center applications which work quite well and, of course, are available for free. If you're not very familiar w/ Linux, you may want to stick w/ one of the Windows versions. I personally prefer Vista Home Premium for media center applications as I find the media center portion is better than what XP Media Center offers. The extra expense of Vista Ultimate over Vista Home Premium doesn't add anything to an HTPC unless you have "home-made" //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

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