I'm going to go out on a limb apposed dto most people.
the primary thing you have to concern yourself with is heat / noise with a HTPC. considering it will be used primarily for multimedia you must somwhow make it virtually silent as not to interfere with your media experience.
one thing to note, you do NOT need super power for this. any media playing itself requires virtually NO resources.
you can run virtually any multimedia (DIVX, DVD, VCD, MP3, wave, you name it) on a 1ghz CPU sometimes even lower. (I know this. I have a 1ghz laptop that does all media formats fine)
so what does this indicate? why blow the money on a fancy CPU that will just run hot as hell and require more cooling
Here's a site to look at that deals heavily with CPU's and parts for low power and small size.
http://www.mini-itx.com/
if you do want to go with something more upscale. a simple AMD a64, or even Sempron since it will run a bit cooler and will definately be cheaper.
Obviously, the less fans you need, the bette,r and the slower they spin the better. and when they're slow, the larger they are the better. lol
I highly THEN recommend this Cooler
http://www.newegg.com/Product/Product.asp?Item=N82E16835118115
it will provide suitable airflow within a case aswell allowing for minimum other fans. it also runs super quiet. I run one on my 3700+ at the slowest setting, can't hear it at all, and it keeps my CPU <35c
as fro video, since it's not a 3d machine. any no fan solution will do. look for one with DVI out cause if you have a HD TV with DVI in it will provide for a great picture