Ok I’ve built a few HT PC’s so I can help you out. I don’t know how many components you’ve gathered already, but just in case you haven’t gotten everything yet:
Get everything from Newegg. Get an dual core processor, AMD’s are at a great price point right now. I’d recommend the X2 4400+. As I mentioned before make sure your video card can do hardware video decoding I belive all of the lower end Geforce 8000’s will do you fine. Get yourself a couple of hybrid tuners for free digital off the air broadcasts and cable. Try and look for ones that can handle QAM. This will allow you to receive your (unencrypted) digital cable channels, but isn’t necessary. I’d recommend you get two or better yet, three hardrives and run them either in RAID 1 or 5 respectively.
Also, go with Windows Vista Home Premium or Ultimate (get the OEM versions from newegg). It comes with windows media center.
And finally, look into a good cordless remote control and keyboard. I have a Gyration remote which also acts as a mouse which saves a lot of time and makes it a lot easier to navigate to say the netflix webpage and watch downloadable movies.