For your HD TV monitor, you will need to purchase a seperate HD decoder (the Sony one that is available in most retail stores goes for $799). With that, you'll be able to enjoy 1080i picture quality.
For your PC to run HDTV, you'll need to buy an HDTV tuner card--the Hauppauge card is the best to get--fraction of the cost of an HDTV or HDTV monitor too.
NOTE: YOU DO NOT NEED A SPECIAL CABLE SUBSCRIPTION to watch view HD broadcasts. In fact, TimeWarner's "Digital Cable" is sort of decieving...they call it digital because the signal travels through their new fiber lines. Most major cities (and some smaller ones) are broadcasting both in HD and analog frequencies right now. What does that mean?? If you live in a decent-sized city, your ABC, CBS, NBC, PBS, and maybe UPN, WB are broadcasting their HD signal!!! Yup, a standard antenna and your HD decoder box connected to your HD TV monitor using the HD component connection is all you need.