Advice on P4 Specs

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I would like to buy another systems using P4, so what specs do I have to get, because I'm confuse in many things:
1. what's the different between Northwood and Northwood A?
2. Rambus cost much and doesnt oc too high, so should I go to DDR, what about the differences between the two when combined both oc and performance?
3. from above, what specs do I get, for the processor I'm using the clock <= 1800 Mhz

thanks in advance

Life just like a maze

 

[Access]

I forgot to mention, the specs that I meant was:
mobo, proc and memory

thanks again

Life just like a maze