Who cares just as long as you run a 64-bit OS, load up on RAM, and have access to fast disk.
Any name brand mobo an with Intel Core i7 processor should do. With CS4 it seems the graphics card is a more important consideration.
Asus M4A79 deluxe, Phenom II X4 940, 64 bit system, 8 gigs of ram because you can and it is cheap.
For future updating, I would use a board running DDR3. The price is dropping rapidly, and if you are running i7 processors, the XMP versions offer advantages.
Exact hardware is not critical with Photoshop, though there are a lot of things you can optimize depending on your application.
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.
