I haven't been keeping up with PC PVR technology for few years now, but the Happauge cards used to be the most widely supported. There are some Windows MCE certified capture cards that are also good (like the ATI one) and have been ported to other software programs, but you'll want to make sure it works with whatever software you want to run.
I'm not sure what the latest and greatest on the HD front is. Up until a year ago, this was a major roadblock for PC based PVRs as the only HD capture hardware affordably available could only record OTA HDTV and would not record HD from a cable box or sattelite box. But I see Happauge has a newer product (
http://www.hauppauge.com/site/products/data_hdpvr.html) which looks to be supported by MythTV.
Having a few years of experience with a couple of different setups, I'd have to confess that I don't think it was worth it. Too much hassle, especially when trying to deal with an external cable box. I'd probably rather just get a ready to go set top box if I wanted to do PVR now. By the time you buy all the little bits and pieces you need to get a PC PVR working (capture card, remote, IR blaster, video card), you're already blowing almost as much money as a set-top box costs.
Just my two cents.