This should be mover to the PC or gadget area, but most quality DV capture cards will utilize a breakout cable that will support a few connection types and then you may need to use an adapter to merge you device to it; so don't expect a consumer item.
What source material are you wanting to go into the card?
What type of PC connection? PCI? USB/External?
You will need a ton of processing and cooling and HDD.
Pretty much a friend is looking for something to capture off his HD receiver in HD quality. So we have been looking for a way to capture with his myth box via DVI input. Usinag an adapter will cut the quality I think.
It sure as heck isn't doable under $200 but the people that have done it are recording HD as good as it comes.
If it won't work directly with the Myth box, I'm sure the MythTV box could either get the stream or the recordings from whatever PC is interfacing the R5000 modded receiver.
The OP didn't mention what kind of HD receiver is friend has, but the r5000 mod will work for most satellite boxes. I use one with 4dtv and record the master HD signals from HBO, Showtime, Starz, and Discovery. Like Shawn said, it isn't cheap, but the picture quality is most excellent. Incidentally, r5000 doesn't work with Myth, but they do have Sage integration now.
If you're friend happens to be using an OTA receiver, then a MyHD card would be an excellent choice and it does output DVI through a daughter card. I think the cost is down to around $224 these days for the MyHD card and the daughter card combo. The nice thing about MyHD is all encoding and decoding is done in hardware, so it puts next to no load on your PC. A very minimal PC can easily handle OTA HD recording with one.