Lets see.... Building a media computer is probably like building one form gaming. Hard drive is only important depending how much space you want. Memory is the most important to me but it depends if your computer will recognize it all and use it (Windows XP and older will not recognize more than 3 gigs I believe Windows Vista and Windows 7 will see all and use all).
Next big thing is the video card. Buy the best you can and it will solve a lot of problems. With the "SLI" using two video cards at one time will also improve the graphics greatly. You can search Newegg or Tiger Direct for the video cards, I like Newwegg. Make sure you read the customer reviews on the video cards.
I built a computer awhile ago with a TV tuner in it thinking I would watch TV, surf the internet, and listen to music. That didnt last long. The TV Tuner was a pain in the butt and conflicted with other things in the computer. You can have a much better computer if you leave the TV out. I think Beemaster hooked his computer into a large screen TV as the monitor.
When I am in the mood to day dream and trying to think of the best gaming computer to build I usually go to "ibuypower.com" and build a computer to see what something I would like would cost and what parts I would actually need to order. Yes I am a gammer and speed on the computer means everything. I put a Raptor hard drive in my current computer, speeds are 10,000 instead of 7,200.
http://www.ibuypower.com/lobby.aspxI believe Dell offers "media" computers. Try looking at theirs and get some ideas and then go order your parts cheaper from a computer store outlet, like newegg.
http://www.dell.com/home/desktops#subcats=&navla=133615~0~4774329&navidc=DT: How fast does it need to be?&navValc=Multimedia - ideal for video playback and multitasking&a=133615~0~4774329
Sound, Most motherboards are coming with 7.1 or better sourround sound. For better sound buy a Soundblaster card at the price you can afford. You will notice the difference in sound.
Below is somthing I did quickly from newegg. I would suggest two video cads and two hard drives raided together. You still will need a case, power supply and other inputs.
GIGABYTE GA-EP45-UD3P LGA 775 Intel P45 ATX Intel Motherboard - Retail
Model #:GA-EP45-UD3P
Item #:N82E16813128358
Return Policy:Standard Return Policy
In Stock
Mail in Rebate Card13-128-358
$134.99 -$5.00 Instant $129.99
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Update14-130-339 EVGA 512-P3-N871-AR GeForce 9800 GTX+ 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card - Retail
Model #:512-P3-N871-AR
Item #:N82E16814130339
Return Policy:VGA Replacement Only Return Policy
In Stock
$134.99 $134.99
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Update19-115-037 Intel Core 2 Duo E8400 Wolfdale 3.0GHz LGA 775 65W Dual-Core Processor Model BX80570E8400 - Retail
Model #:BX80570E8400
Item #:N82E16819115037
Return Policy:CPU Replacement Only Return Policy
In Stock
$167.99 $167.99
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Update20-134-863 Kingston 4GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Desktop Memory Model KVR800D2N6/4G - Retail
Model #:KVR800D2N6/4G
Item #:N82E16820134863
Return Policy:Memory Standard Return Policy
In Stock
$146.99 $146.99
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Update22-136-073 Western Digital Caviar Blue WD5000AAKS 500GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
Model #:WD5000AAKS
Item #:N82E16822136073
Return Policy:Standard Return Policy
In Stock
$55.99 $55.99
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Update29-102-006 Creative Sound Blaster X-Fi XtremeGamer 7.1 Channels PCI Interface Sound Card - Retail
Model #:70SB073A00000
Item #:N82E16829102006
Return Policy:Standard Return Policy
In Stock