I want to use Linux as primary OS in my new desktop.
So I want to know which motherboard is most competibile with Linux?
I want to use PentiumD or Athlon X2(AM2) cpu.
Any sugestions?

i'm on p4 but if you're after games or some programs with quick hardware response, go for athlon. that's all i can say.

Hell no. Wait for the new Intel CPUs to hit the market, once they come out get one or if your budget isn't that high, aim for an Opteron. They should drop in price as the Conroe core Intel CPUs will push all but the top Opterons back to the mid-range market.

As for motherboards, I've heard so many varying opinions it's not funny. Some say Asus is great, some say it's cheap crap. Some say MSI is great, other say it's cheap crap. Best advice is to look for one that has everything you're going to want plus room to upgrade. Then first search for reviews, comparisons and read the comments on the retailers site to see what others have to say about it then secondly, check the chipsets onboard for linux compatibility. Once you are well researched you'll be in a good position to make your own choice, that's how I picked out the parts for my current machine

well that's something new. we can't really know how good systems are now.

Regardless of which processor you choose, you're going to want to look here for Linux compatibility and user reviews.