With the help of this forum I built a silent PC mainly to play music on my HT back in 2007. It has:

Case Silverstone LaScala LC-20M
Mother Board Asus M2NPV-VM -- mATX
CPU AMD Athlon 64 3500+ Lima 2.4GHz
Memory 2 gigabytes DDR2 Memory
Power Supply Seasonic S12 430Watt PSU
DVD-CD Player Generic DVD CD Player
Case Fan PCExtreme 250mm 12 VDC Fan
CPU Cooler Thermalright ultra 90A
Hard Drive Western Digital Scorpio WD1200BEVS 120GB
Keyboard Gyration Cordless Keyboard
Mouse Gyration Cordless Air Mouse

The MB was selected because it has component video out and the CPU was selected for minimal power consumption.

I would like to upgrade the motherboard to one that has HDMI output and upgrade the CPU to run Windows 7.

Back then Intel processors hogged power thus AMD. I assume the new Intel processors use less power. I've read the post on recommended MB but not much is said about power consumption and MB features.

Can anyone recommend a CPU and motherboard for this application? The PC will play music mostly. The music is on a high speed server in another room and this PC is wired to a CAT 6 network. I would like to use HDMI output from the MB. I'm feeding signal to two HDTV's, one next to the PC and one is upstairs and HDMI cables have been threaded through the walls, thus the HDMI output is needed. It will also play some movies again from the server.

If power consumption is important to you, go with an intel mobo, since you current is a micro atx mobo maybe something like DH67BLB3 or DH67GDB3 (even has displayport). I haven't had any issues with both intel mobos im using, but i seen some comments about some memories causing issues, so if you go this route check reviews for what people are already using and go with that to avoid incompatibilities.

For CPU, for music and 1080p playback, a Intel G530 should be more than enough, comes with intel HD2000, now if you want a slightly better igpu, then consider Intel Core I3-2105, the lowest cpu that comes with intel HD3000.

Back when I built this quiet PC I was switching from an eMachines with an Intel CPU desktop that was implemented to play music (I tossed the big mechanical CD changer). The eMachines CPU fan would whine away making awful fan noise it seemed usually during quiet passages of music. The AMD CPU was recommended because of low power and heat. The CPU fan never comes on in this home built unit.
So power consumption is only a concern from a fan noise perspective. The PC is cooled by two large 200 MM low speed fans so air flow is not a problem. They run on low voltage and cannot be heard.

http://www.silentpcreview.com/forums/viewtopic.php?f=14&t=42579&p=362186#p362186

I read some reviews of Intel HD 3000 graphics and I would, of course, like as much performance as possible (I might even play direct downloaded HD movies on it, not just music). The only issue is I do not want to hear the CPU fan. The AMD CPU and mobo was selected for low power specifically to reduce CPU heat so the fan would not come on and performance was sacrificed. With the 200 mm fans the CPU temp of the AMD never exceeds 41 degrees C when playing music and the CPU fan never comes on. With a stress test the CPU will get to 55 degrees C and the fan comes on but this never happens during normal use.
The power consumption of the AMD Athlon 64 3500+ Lima 2.4GHz is about 45 watts and if I read the Intel core i3 spec sheet correctly it consumes about 65 watts. If memory serves me I think the Intel CPU's of the day (2007) were well above 100 watts. I would hope to have the CPU fan either never come on or very slowly.

What do you think?

Streaming music doesn't take much processing power. Here's an SPCR article comparing various Intel and AMD CPUs. Figure your music streaming load is somewhere between idle and H.264 playback - call it ~45W DC system power. You can probably use the Thermalright Socket 1156 bolt thru kit to adapt your Ultima 90 for socket 1155 (check with them, first - or maybe another user can pipe up).

The H67 mobo is a good fit. Intel for lowest power or Asus for good fan control. If you want HD 3000 graphics, then go for the i3-2105. It's a solid choice amongst HTPC builders.