I plan to build a new HTPC using the Silverstone GD08 case. I want it to be quiet with good graphics and sound performance. I favour ASUS mobo's, not interested in overclocking or games. Best mobo (with displayport and HDMI coonectivity) and CPU recommendations would be welcome.
If you're building exclusively for an HTPC and don't plan to game than I'd recommend any Ivy Bridge i5 processor so you at least get HD2500 graphics or i7 if you want HD4000 graphics. Unless you plan on doing serious gaming you shouldn't need a dedicated video card. Anyway, the i5-3570 can be had for $215 on newegg (Link: http://www.newegg.com/Product/Product.aspx?Item=N82E16819115233 ) and it is a very capable processor that is rated for 77W maximum power draw. If you want/need HD4000 graphics then the cheapest i7 processor you can get is usually the i7-3770 and it can be had for around $320 (Link: http://www.newegg.com/Product/Product.aspx?Item=N82E16819116502 ).
As for motherboards, I'm assuming this case only fits micro-ATX. On that note, given your description there really is only one motherboard to go with and that is the ASUS Maximus V Gene LGA 1155 Intel Z77 Motherboard (Link: http://www.newegg.com/Product/Product.aspx?Item=N82E16813131830 ) and it just happens to be at the $200 sweet spot. Gigabyte also has a G1.Sniper motherboard in the micro-ATX form-factor but as you're building an HTPC I don't think it'd work as well considering the Sniper motherboards are made to have less features but overclock well.
Also, regardless of what you go with, make sure to get a motherboard that uses the Intel Z77 chipset; this far into 2012 there's really no reason to buy a motherboard with any other chipset.
EDIT: If the case is mini-ITX I'd recommend this ASUS motherboard: http://www.newegg.com/Product/Product.aspx?Item=N82E16813131840
for the processor, id get something like a i5 3470
for the motherboard id get either something cheap like a msi h77ma g43 or the gigabyte sniper m3. the sniper m3 has very very good onboard sound.
for media, you can do fine with the onboard stuff or go with a 6670 which has very good graphics for movies.
remember the gd08 is the size of a regular sound reciever