I recently built a new HTPC for myself, however, since I also have a primary PC this HTPC is strictly for media with gaming as secondary (maybe).
You will probably want this entertainment PC to be quiet unless you like listening to fans when watching movies and listening to music.
The decision to get a HTPC style case to fit in with your TV and other media components (if any), or a regular PC case like the Antec P182 is up to you. However, you cannot simply rely on the case to keep the noise inside it. All components will ultimately determine how loud your overall PC will be.
For the PSU, I highly recommend the Seasonic S12 series. If you want a modular PSU, then consider the Corsair HX series (made by Seasonic). Seasonic's M12 series is modular, but it adds an extra fan (2 in all) it is still quiet, but not as quiet as the Corsair HX which only uses one fan. Seasonic S12 PSU can achieve up to 88% efficiency (based on the load). The higher the efficiency the less electricity is wasted as converted heat. Below is a list of Seasonic S12 PSUs"
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Description=Seasonic+S12
For the CPU I would recommend the C2D E8400 which is 3.0GHz (stock speed), and is easily overclockable if you want. You could opt for the Q9xxx series, but not very games are even optimized for dual cores yet.
I recommend a large heatsink to keep your CPU cool. In my HTPC I have a Scythe Ninja+ passively (no fan)cooling my E6600 @ 3.0GHz (both from my old PC); after encode videos for a few hour it maxes out to 58C using Core Temp to measure the temperature. There is an all copper version of the Scythe Ninja which just can out this year. Note: I am sure when summer approaches the CPU's temp will increase.
For hard drives I recommend Seagate Barracuda 7200.10 ST3250410AS 250GB 7200 RPM 16MB Cache for your OS and some unimportant data and games. It is a very slim SATA hard drive about 2/3 the height of a typical hard drive. It uses a single platter to store data so that means it is quiet and produces less heat.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148262
As for data that is important to you I recommend you store them on a separate hard drive. I would recommend the Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB. It is really quiet because it use two 320GB platters to store data. The few platters a HD has, the quieter it is. I use these in my primary PC.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218
If you want even more storage capacity on a single drive (1TB) then I would recommend the Western Digital Caviar RE2 GP WD1000FYPS 1TB 16MB Cache. Very quiet, but also expensive since it an "enterprise class" hard drive. Meaning it is made for businesses where hard drive failure is not an option within the warranty period (5 years). I have two of these in my HTPC to store my videos and music.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136206
Speaking of hard drive reliability, some people set up RAIDs and/or use external hard drives to back up data. RAID is not considered a backup, it is for redundancy (that's the 'R' in RAID). Redundancy means if one drive fails you can still access your data, but should another drive fail you will not be able to access your data anymore. I was originally going to do RAID 1 (mirror) which simply writes/reads data to both RAID 1 drives, and get an external hard drive to back up my data from time to time. But I think I will drop RAID 1 and simply backup my data to external hard drives.
Since you intend on playing games on an HDTV I will assume you want to play @ 1920 x 1080 resolution, I will simply recommend a 8800GTS. Get any brand you want, but EVA generally is better since they allow you to add on a third party cooler without voiding the warranty. The 8800GTX is more expensive and there are fewer of them which makes sense ince the release of the GeForce 9800 series.
Speaking of the 9800GTX it doesn't perform all that much better than the 8800GTS and consumes a little more power as well.
To cool the 8800GTS I would recommend the ARCTIC COOLING ACCELS1 Rev 2 VGA Cooler which can passively cool the 8800GTS. WARNING it is tall, once attached it rises about 1.5 inches higher than the video card.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186016
You can also get the optional fans if you think the temps are too higher, especially after overclocking.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186021
In case you're wondering I have an ECS 9600GT that is passively cooled by an Artic Cooling AccelS2. But I haven't used it to play any games yet.