I currently have 2 x 256 Crucial cas 3 ram at 143 mhz. I am going to get a stick of 512 mb crucial. I would like to stay at 143 mhz and they have the cas 2 and cas 3 versions at the same price. What shoud I get? Would cas 2 overclock (and change into cas 3) higher or do i have to use the cas 3 ram?