You really need to tell us the motherboard model as to what variables are best, what its front side bus is etc ie some people try to put in 1Gb sticks into a memory channel that can only take a max 512 mb stick of ram and therefore the 1Gig will only run at the max memory channel speed.
FSB, Latency, Dram Timing Etc are all pertinent information which can be found if you give us your MOBO model
give us some more info
