The real critical choices are the motherboard, CPU, and case. The motherboard will be determined/will determine the CPU you use, and these will in turn make your power supply choice. The case will have to be able to accept the power supply and motherboard, you will have to have adequate ventilation, etc. After that, it's a matter of what sound, video, CD, HD, and floppy, which doesn't amount to that much trouble. Your RAM will be dictated by the motherboard as well.
I would spend my time looking at the specs of motherboards first.