Gigabyte GA-8PE667 help/memory!

Let me start off saying so far this has been on great mobo! I put a P4 1.8 and wiht a Pc2700/512 it is running at 2.4. I'm thinking of adding more memory do I put two more of the same, or Pc3000/512, or maybe two of the Pc3500/512. I'm new at this and as I try to read the out dated user's manual wrote in #@$%^^&&*%$@ @# ^& Helpme Helpme Helpme I cry!