I have 2 gb of OCZ Platinum Revision 2, pc2-6400. When I get Win 7 and use the 64bit OS I may want to get 2 more gigs of memory. As long as I stick with pc2-6400, what are the consequences if I get memory with different latencies? Would there be a NOTICEABLE difference in performance? Would it harm my system in any way?
For the best results you can manually set the parameters for memory to match the weaker chip's specifications.
Since they're both pc2-6400 the difference would most likely only be visible in a program designed to measure bandwidth and highly unlikely you would notice it.
It will be noticeable going to 4 gigs in any programs able to address the additional memory.
The most relevant issue you might want to Match is the voltage of the chips by the manufacturers listing.