When it comes to ATSC - all "capture" cards are doing is tuning, doing the error correction (in hardware), and literally writing the decoded MPEG program stream to disk. It requires virtually no cpu or horsepower to speak of.
Back in the bad old days of NTSC a capture card had to decode, construct each frame, encode to MPEG-2 or whatever, then write that to disk.
I use MythTV based on Ubuntu 10.10 (mythbuntu its called) with 2 cards (obsolete, but usable hybrid tuners) Hauppauge 1600 and Hauppauge 1800 (one is PCI one is PCI-e).
Records perfectly and in beautiful HD. MythTV even commercial skips for you and makes it simple for you to automate encoding with something like mencoder or manually with Handbrake if you wish to archive.
Worth noting that while WMC gets program data seemingly from the ether - (though likely its from the MSN TV listings) - others must still pay "someone" for the program data. It "CAN" be gotten from EPG over the air, but that is unreliable at best, completely wrong and or missing at worst. In my area I can count on 4 hours of EPG and thats all. SchedulesDirect is $20/yr.
With my setup - E8400, 4GB DDR2 1TB disk (single 7200rpm for buffer and OTA recordings) - it rarely goes past 5% cpu time (watching top remotely) when recording 2 1080i streams. I'm certain at some point the disk I/O starts to bottleneck, but it seems 19mbitx2 isn't really all that taxing yet.
Watching 1080p BR rips however - THAT takes some CPU. I haven't actually gotten the MythTV video card acceleration working well yet - its called vdpau - and does some odd things with my video card.
Aharami - that is a totally different situation. You have no "reasonable" option for a DVR in the basement. Cable card cards are now shipping but the price will put you through the roof. Ceton makes the only CableCard tuner for consumers in existence, and it's $399.
http://www.cetoncorp.com/buy.php
The next possibility is to just record what comes out of the Comcast DVR. No HDMI device will allow you to record what is displayed over it. Thats part of its design. There ARE HDMI capture cards, but way over $1000 and they don't do audio. (BlackMagic comes to mind).
SO Component recording is possible, the Hauppauge HD-PVR will record anything Component - so you can record 480p (likely what comes out of the Comcast DVR on the component cables) or more if thats what that box will supply. It's integrated only in GB-PVR and SageTV from what I understand.
Next is any Svideo/Composite solution which can be done with any $20 analog card.
So lets re-examine what is desired:
You want to "watch" what is coming down from the Comcast DVR? Maybe you only need to connect the Component cables to it? It's feasible that you could use HDMI for one set and Component for another but that depends on the box.
That's it. Yay Comcast!
Noting that the Ceton card should and will work in WMC7, as Robert Heron has reported here:
http://heronfidelity.com/news/ceton-infinitv-4-update/
So for $399 and a PC + another CableCard from Comcast is likely the most economical resolution for "HD Cable in the Basement"