it really depends on how much you're willing to spend, there are so many good processor + motherboard combinations out there, it's difficult to make a recommendation without knowing this.
a quad-core really isn't necessary, though it will be futureproof. cs4 is now 64 bit, so it can take advantage of the extra cores; though a hardware review site did come tests on dual-, tri-, and quad- core and there was no real performance difference between tri- and quad-core using photoshop. the same is true with newer games that support the 64 bit architecture.
tri-core processors have the advantage of being cheaper than quad-core though they are currently only available from AMD (their phenom ii range). i, personally, am using a phenom ii processor and am more than happy with it. saying that, i would wait until a few more people have posted their opinions, and then i would suggest you do some research on the recommendations to find one that is suitable for you.
with regards to the motherboard, it would depend on what other components are going to be used, and which brand of processor. i tend to choose a processor first, before looking at what motherboards support it and have the features i am looking for.