If the HTPC is only for playing movies, audio, TV, viewing pictures, and perhaps some web browsing or other light work. Then you probably don't need such a powerful PC.
I built an HTPC around an ATOM N330 ION using a Zotac ITX motherboard.
I used a tiny case as well from Antec
Don't be put off by the ATOM, coupled with the ION graphics which are integrated into this board, it can cope with 1080p HD video with ease.
With the case above, you can only fit 2.5 inch drives and a slimline BD player. I used an SSD as i stream content to the HTPC across the network so I didn't require a large HDD in the HTPC.
It draws around 35W and is completely silent.