Well, first and foremost, processor speed is the leading factor in PC performance because without the extra speed of a processor, 1333MHz RAM won't speed up your system. It is best to have a system with a high processor speed and then focus on memory. Any DDR2 800MHz memory will give all the performance you need, but if you wish to go higher, DDR2 1000, 1066, or 1200 will give you slightly faster performance.
Basically memory won't boost the performance of your system if your CPU is lacking, because all processes must be fed through the CPU. If your memory is top notch and sending packets of information at high speeds, but your CPU is still a low 1.8GHz, with low cache, it won't take advantage of the memory speed boost. It can't. It will just slow things down.
So in my opinion, it's CPU first, then Memory. It's like comparing your brain to your arms. You can have arms that move faster than bullets, but if your brain is slow and can't function, the arms won't be able to meet their full potential either.