I just built my first HTPC about 3 months ago. I am a newbie at this and I am learning tons of things about CPU's everyday. A few things from my experience. First and Foremost, purhase the Assassins Guides to help with your build. Saying this would help tremendously would be an understatement. If Rome had a guide like this, it would have been built in a day. Secondly is this HTPC going to replace your cable's DVR boxes and will be used with extenders so that other TV's in the house will use it? If yes you may want research an alternative TV Tuner.
Secondly, as a newbie, you are going to read tons of post on this board where folks will say, you are only going to need this amount of that. Such as "G640 CPU would have more than enough power to do want you want an HTPC to do", or "I got my HTPC working with just 2gb of ram." These statements might all be factualy, but you are going to have to dig deeper to see if these items would work for you. For example 2gb of Ram could work for your HPTC, but lets say you have 5 TVs in your house and you plan to use 5 XBOX 360s as extenders( I know this my be a some hyperbole) 2gb or ram simply will not work for that set up. You would need more like 8. When confronted with this, i opted to go with the bigger is better approach. I bought 8gbs of ram form my build. I purchased the i3 3225 CPU, so know I just feel better about my build and I don't worry if I shorted myself.
Lastly the build part of your HTPC is only 20% of putting the HTPC together. 80% of your time will dedicated to adding and tweaking the softward so this baby hums.