Re: Which CPU?
It's whatever you can afford. The bigger price you pay, the better performance you get and the Athlon 64 X2 dual cores are almost twice as much as the single core Athlon 64's. Don't worry about the M2. It won't be here until late next year.
If you're going to get a PC built, current specs use Socket 939 with an Athlon 64 CPU of some type. I personally feel that the X2 will benefit you because you want to do gaming, music editing, and who knows what else. With dual core, you can do both at the same time and no slowdown will occur.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?