After stalking the forums for a while, it's time for my first post here! I'm been an enthusiast reader of the SCPR site and forums, I really like some of the creative solutions people have come up with here!
This time I have a question of my own. I run a small server/HTPC in my room, currently an dualcore Atom. It's a great little pc for what i'm using it for, however i would really like some more juice in the CPU department. I decided to bought myself a Celeron G530 and I got an Asus P8H61-M rev 3 (m-atx) motherboard for cheap. This motherboard is not really the motherboard i want for my server, but i could get it incredibly cheap and i thought it would be a nice starting point while i search for a new board. I prefer an m-itx board, which is as energy efficient as possible, so i probably an Intel or MSI board. So right now I'm searching for a new motherboard but i lost track of what to choose. There are a lot of chipsets eligible like 55,61,67,77 and i lost track. One of the challenges is combining a decent amount of sata ports, and SPIF out, ánd HDMI at a competative price. Here are my demands for the new board:

The motherboard should have:
* Socket 1155
* M-ITX form factor
* Lowest power consumption possible
* HDMI out
* SPDIF (either optical or digital coax, both are fine) (since my receiver has no HDMI input)
* Have at least 4 SATA ports
* The possibility to increase the number of SATA ports by some extension card

I know I'm asking a lot for a small and affordable board, but does anyone have a good suggestion for this? Many thanks in advance!

Intel's DH77DF is the board that meets most of your requirements. The only downside is that it is relatively new and so you won't see any discounts on it for a while. The main benefit to going with the latest 7 series chipsets is native SATA 3 and USB 3 support. Older chipsets require additional controllers to gain those features and those implementations may present issues with performance or software support down the line.

For your future SATA port needs, you can take advantage of the mini pci-e slot on found on the DH77DF (and other recent Intel desktop boards) and use this mini pci-e SATA controller card to gain two more SATA ports.