As I mentioned, after proper OSS support, Stability is the main concern.
I can imagine a windows gamer don't care much if it crashes every once in a while. to me, that is unacceptable. Stability > speed :)
I tried googling for a vendor list that actively supports OSS. Realtec came up as one of the few that really does good on that side (together with 2 other asians, but that was more on the subject of firmware (for wifi) that I found). There should be a nice comprehensible list where vendor Linux support should be listed ...
I also failed to point out, one manufacturer you'll want to avoid at all costs would be Foxconn. Foxconn PR has told Phoronix in the past that they do not support Linux, and refuse to even let Phoronix test them so we can draw our own conclusions.
On DFI's old support forum they had some talk of running Linux on DFI's motherboards. It wasn't much, but the DFI contractors on that forum did go through the process of installing a Linux distribution (don't remember which one), giving details on what they did, and opening up a discussion around the topic that turned into a couple of good-sized threads. It was obvious that their main interest on DFI boards was not running Linux, but at the same time they realized there were some in the community who were interested in that. It didn't go much further than that, and one thing I tried to do occasionally on that forum is open up more discussion about Linux. I don't have a clue how much DFI supports Linux on their devices, but they do provide Linux drivers on their website.
What about FIC?
I used to have an FIC board for my AMD K6-2. Though I had to solder on it a bit to keep it going longer, and I blew up the PS2 port (by shortening out a keyboard while tying to replace the leds) it's still going strong. My mother was using it up until this christmas. So it was in active service as a desktop, a server and a desktop again for almost 10 years now (damn that makes me feel o l d). Though the MVP-3 chipset had some pci latency issues in windows with the Creative SB Live! (also still have that card in active service in my desktop lol)
With FIC bringing out the OpenDokumo or something phone running linux, it's at least sure they are linux friendly.
Well their mobo section is ... very light, and they don't even have some rescent motherboards (even older ones, like KT880 based for example)
They do have some intel's though. AM2 they don't even have at all. So scratch that idea. They seem to be more into products then components these days