Maybe this is old news, but I was surprised how well my new HTPC build works. Specs for completeness:
Case: Antec Fusion Black w/430W power supply
Mobo: Asus P5N7A-VM (GeForce 9300 on-board video)
CPU: Intel E8400 C2D
Heatsink: Scythe Ninja Mini (juuuust fits, yet surprisingly easy to install)
RAM: 2x2GB Crucial DDR2-800
HDD: WD 640GB Green-power
I was a bit worried about the heatsink; I've never used an aftermarket heatsink, because of warranty issues, but I figured if the E8400 died I could upgrade to a Quad.
Heat is not an issue - max cpu temp is 40C under load according to the Asus Temp Monitor. Even though it isn't necessary, I did use the CPU fan that came with the Ninja, because the motherboard complained about "missing CPU fan!!!" and it was annoying.
The system as a whole is too quiet to hear unless I'm sitting right next to it with no one else in the room, and the built in video is powerful enough to play anything I throw at it, but I don't have a Blu-Ray player so I can't testify to that. It plays DVDs and my H264 DVD-rips without hiccup, though, and it is CHEAP and QUIET.
AND it's worth 1700 PPD for Folding@Home! Highly recommended.