What is your purpose for this build and what are the parts that you currently have that your going to use for this build??
Intel and nVidia chipsets haven't been all rosy as jpdykes has stated. Currently the X-fire mobo's have been quite a bit more stable and reliable over some of the nVidia offerings. This is not to say AMD chipsets haven't been all rosy either, but I'd consider a p45 based mobo. The 4870 is a nice GPU, depending on your requirements. the q9950 is quite expensive and I'd usually recommend something cheaper, but since I don't know what your purpose of this build is, I'll hold off on recommending any CPU's.