You'll need to get an HD-PVR (http://www.hauppauge.com/site/products/data_hdpvr.html) and a cable box that outputs via component if you need something right now. Otherwise with a basic tuner you might as well ditch the cable and just put an antenna up on your roof to get OTA HD because you'll probably only be able to get the same local channels in clear-qam anyways.
The new Ceton cable card tuner in Q1 2010 could also work if you're willing to wait, but I have some doubts about it being able to record some channels.
You have several tuners to choose from. It depends on what type of signal you receive (include OTA captured via antenna or cable or dish) and how many tuners you want. Several nice dual tuners are listed on the infamous "page 85" tables.
The HD-PVR requires you to have a set top box and you do not need an internal tuner in your PC. Going this route allows you to capture many more channels (including premiums).
That being said, the cheapest route is a single tuner card ~$50. Dual tuners will double that. Capturing the premium movie channels requires monthly rental of a STB and an HD-PVR at ~$200.