Any name brand mobo an with Intel Core i7 processor should do. With CS4 it seems the graphics card is a more important consideration.
Get a system with a reasonably fast CPU, and at least 2GB or 4 GB of RAM.
Fast disk is, IMHO, less important than lots of RAM. If you use Windows,
and deal with very large images (>150 megabytes), or routinely open a lot
of images at the same time, look into a 64 bit system, and upgrade to CS4
64 bit.