What do you want to do with your HTPC and what capabilities do you need in audio output and refresh rate terms?
1. If you want to watch streamed online stuff (iPlayer HD etc.) then you need a decent CPU - as not all streaming can be hardware accelerated in the GPU.
2. Do you want to re-encode video on your HTPC - convert DVDs or Blu-rays to a form you can play on your iPhone or iPad for instance? If so, again, a decent CPU is needed.
3. If you want to stream lossless audio (Dolby True HD, DTS HD MA) from Blu-rays via HDMI to your amp, then you need something that supports that.
4. If you want to watch 1080p Blu-rays on a display that supports "24p True Cinema" or similar you need a GPU or IGP that supports 23.976Hz refresh rate (otherwise you're likely to get a repeated frame stutter every 40" or so)
Of the integrated solutions :
AMD Zacate meets 3. and 4. but not 1. and 2.
Atom + Ion meets 4, doesn't fully meet 3 (but can decode to PCM 5.1/7.1 with some limitations), and doesn't meet 1. and 2.
CULV + Ion meets 4, doesn't fully meet 3 (but can decode as above) and does a lot better with 1. and 2. (but is rare)
i3 Clarkdale meets 1,2 and 3 - but struggles with 4.
i3 Sandy Bridge meets 1,2 and 3, and can almost do 4 with some limitations - but not perfectly.
Skt 775 Core2Duo + nVidia 9400 can meet 4, 1 and 2 and not fully meet 3 (but decodes to PCM 5.1/7.1) - but are rare (possibly still Mini-ITX available)
If 3. and 4. are important AND you want some CPU grunt - you're looking at an ATI 5450/6450 or better (or an nVidia equivalent) graphics card as none of the on-board stuff will suit. Then you can chose an AMD or Intel CPU purely for performance (and power consumption and ignore video / audio issues)