IMHO main question is whether you wanna do any gaming on it.
second question is whether you wanna run it for downloads or torrent or for any reason you want it to run >4 hours everyday.

because right now, even the cheapest dual core CPU is capable of decoding full HD. with the exception of perhaps the lowest powered atom/celeron dual core.

some people die die wanna have 6 or 8 core in their htpc without realizing it'll easily cost them an extra 4,000W a day to run it.

If your home theater system supports HD audio (DTS HD MA, Dolby TrueHD), make sure you go for graphics card that support HD audio (eg GT 640) or onboard HDMI (eg Intel NUC) that can support passthru to your amp.

If your amp doesn't support HD audio, optical out will do the job nicely.

you planning on downloading torrent using the htpc?
if yes then get a cheap dual core processor. not necessarily an i3. a dual core pentium with a graphics card capable of hdmi bitstream might be better if you're into audio. refere below. cheaper electricity.

or if you don't wanna run it on days end, can always consider an i5, i7 or AMD multicore solution.
i do agree that onboard graphics while could bitstream hd audio, do sound lacking. blame it on jitter. as onboard graphics usually extract their clock frequency from the main processor clock, the clock sync mechanism is usually not good. we've tested side-by-side between onboard solution from intel and dedicated solution from ATI graphics and the dedicated graphics do sound better.

but the great thing about htpc is that it's upgradeable. and the upgrade path is freaking cheap.
if you find the onboard graphics to sound lifeless, you could always add a dedicated card. an ATI graphics solution that could bitstream hd audio could be had from ~RM100 (HD5450). better card from HD6XXX and HD7XXX series could be had for around ~RM200.