Yeah my old Pentium 4 3.0GHz with GeForce 9500GT 512MB (less than 50 dollars) can display 1080p though I don't recommend a P4 CPU at all. I think as far as PLAYBACK is concerned, any dual core CPU with an up to date graphics (including relatively low profile ones) will do the task without an issue. And "recording" TV shows itself is even less of a concern than than playback is.
For the video processing part, meaning when you record TV shows and transcode them to burn Blu-ray DVDs, what is MOST important is to make sure that whatever conversion soft you use supports GPU transcoding caz it is a lot faster than CPU alone transcoding almost no matter what. If you have an Nvidia card, you wanna choose a card with CUDA technology AND software that supports Nvidia's CUDA transcoding (such as TMPGEnc Xpress 4.0). If you have a Radeon card, you wanna choose a card that is compatible with ATI Stream and a software that utilizes ATI Stream transcoding (such as ATI Avivo video converter). Of course, AFTER you use those GPU transcoding technologies, the better the CPU / the better the video card, the faster the transcoding will be, but compared to the difference between CPU alone vs. CPU plus GPU transcoding, I have seen benchmarks showing that differences are not that large.
So, I think if you can spare money or you really do the transcoding on a daily basis, 65W triple / quad core and Radeon 4850 / 4870 or equivalent Nvidia, but if you wanna save money, 45 watt dual cores and pretty much any up to date card.
I think you should get a separate graphics card or choose a board that has Radeon or Nvidia onboard graphics (like the one mrjspeed posted). People here told me repeatedly that even a cheap Radeon or Nvidia does 10xxxxx times better job than the latest "Intel" onboard graphics.
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Yup, if you choose this motherbaord, there's an Nvidia graphics with the board, so, no need for a separate one unless you feel the card is not good enough for whatever reason. But it should be more than enough for watching Blu-ray movies etc.
I think your choices of HD depends on how much you would like to store before burning stuff onto disks or putting them on external HDDs.. :D but my HTPC has a 1TB HDD.
Your choice between 65W triple / quad core vs. 45 W dual cores has to do with your budget. If you think you are gonna do lotza transcoding or want to be careful, then 65W triple / quad core, but dual cores are enough for "basic" HTPC needs.
DDR2-800 and up RAM, 2GB minimum if you don't use it for anything other than watching / recording TV, 4GB is better if you are gonna surf the net while transcode TV shows etc. It depends on your OS. When I'm on Vista Home Premium x64, my RAM usage often goes above 2GB.
*Edit* I found this info. Maybe helpful in figuring out what is minimally necessary for Blu-ray playback. But keep it mind that XP requires a lot less RAM than Vista.
http://www.digital-digest.com/articles/PC_Blu-ray_HD_DVD_Ready_page2.html
Core 2 Quad Q6600 2.4GHz, CORSAIR DOMINATOR 4GB (2 x 2GB) DDR2 1066 (PC2 8500), SAPPHIRE Radeon HD 4850, GIGABYTE GA-EP45-UD3P ATX, PC Power & Cooling S61EPS 610W, Western Digital Caviar Black 1TB HDDs, LG 22X SATA DVD Burner with LightScrib, Winodows 7 Professional x64, Antec 900 Case