Short Answer

Minimum System Requirements:

• 1.0 GHz x86 or x86_64 Processor
• 192 MB of system memory (RAM)
• 2 GB of disk space (Frontend Role)
• 20 GB of disk space (Backend Role)
• Graphics card capable of 1024x768 resolution
• Supported TV Tuner Card (Backend Role)

Recommended System Requirements:

• 2.0 Ghz x86 or x86_64 Processor*
• 1024 MB of system memory (RAM)
• 10 GB disk space (Frontend Role)
• 80 GB+ disk space (Backend Role)**
• nVidia 128MB Graphics Card w/ TV-Out or equivalent***
• Supported TV Tuner Card (Backend Role)

* A 3.0 Ghz processor is recommended for frontend installations using HD video due to the increased load upon decoding. A lower end processor can be used however with multiple functions on a single machine (ie - Frontend/Backend Install) it is not recommended.

** 80 GB of disk space allows approx 70 GB of usable space for media. However with HD Format it is recommended to use 160 GB HDD or larger.

*** Although nVidia is recommended, ATI, VIA, or Intel graphics cards are supported and will work. Currently, nVidia provides a higher level of support. Also the graphics recommendation is based on HDTV capable output processing.

Note: A good estimate for converting hard disk space to hours of recordings are as follows:

• Standard Definition (Analog) 2.2 GB / Hour
• High Definition (HDTV) 7 GB / Hour

Long Answer

Long answer:

CPU: Lots of different ideas here. Simply put. Using a hardware encoding capture card, you need around 800mhz for watching standard definition and recording another (or watching live tv). You need 3Ghz for High-Definition.
More in depth and complicated answer

* A PIII/733MHz system can encode one video stream using the MPEG-4 codec using 480x480 capture resolution. This does not allow for live TV watching, but does allow for encoding video and then watching it later.
* A developer states that his AMD1800+ system can almost encode two MPEG-4 video streams and watch one program simultaneously.
* A PIII/800MHz system with 512MB RAM can encode one video stream using the RTjpeg codec with 480x480 capture resolution and play it back simultaneously, thereby allowing live TV watching.
* A dual Celeron/450MHz is able to view a 480x480 MPEG-4/3300kbps file created on a different system with 30% CPU usage.
* A P4 2.4GHz machine can encode two 3300Kbps 480x480 MPEG-4 files and simultaneously serve content to a remote frontend.
* I think I could watch (not watch and record at the same time) High-Definition with a AMD Athlon 2600+ and an nVidia 5500. I never got to test this though as I built a new Athlon X2 3800+.

Hard Drive: This depends on what you want to record, and how much you want to keep at a time. I've seen around 1.7-2GB per 30 minutes of Standard Definition from a PVR-150. 6.4-7GB per hour of High-Def.

RAM: 256MB should be the bare minimum. I wouldn't build a system without at least 512MB, and would probably lean toward 1GB.

Video Card: This kinda depends on what you want your system to do. I use an on board Nvidia 6200. I connect to my TV via VGA. I can do HD fine on this card with XvMC activated and bob 2x. I have also used a Gigabyte Geforce 7300GS with comes with a dongle for Component out and also svideo out. IMHO, stay far far far away from ATI. Nothing good can come from that. If you already have a video card and still need a way for TV out, I believe that the PVR-350 has video out via S-Video.

Sound Card: Unfortunately, I just use on board sound because I don't have a nice card here. I'm looking for suggestions on this. (they need to work nicely)

Remote: I recommend the Windows Media Center (MCEUSB2) remote. It is a great remote and if you don't have one, then get one. And if you have one, then buy another and give it to a friend. If all your friends already have one too, then build another frontend and buy another MCEUSB2 for it.

Tuners:

* Standard Definition: PVR-150, works OOB, has hardware encoder, comes in low profile. The PVR-500 has 2 encoders on a single card, but doesn't come in low profile.
* High Definition: HDHomerun. Dual HD tuner and it connects via ethernet so you can free up a pci slot. If you want something internal, then I recommend the pcHDTV 5500. Works great OOB, high quality, also has an analog tuner, but doesn't have a hardware encoder for the analog encoder. (FYI, High-Def doesn't need a hardware encoder because it comes already in mpeg2ts)