Re: Which CPU?
Socket M2 adds better performance and as you mentioned, the DDR2 will help with that too. It's also suppose to utilize virtualization technology in which it adds hardware support that will allow a system to run multiple operating systems simultaneously. Cool huh? Again, it's still too early to tell what they're actually planning to do. There's not a whole lot of specs on that yet because we're still far away from the actual production of it. I don't think we'll see the M2 until around fall of next year.
If you need a PC now, than you can use the Socket 939. If you're willing to wait a whole year or more for the Socket M2 to come out and possibly pay for a higher price component, then you'll have to wait it out without a PC in the meantime.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?