what do these mean, and how should I use them? I remember hearing you shouldn't put PC-under-cpu-clockspeed in, is this very true or should I put 256mb pc133 into my 384mb ram computer that is in desperate need for more ram? (I have PILES of ddr2 but its useless ) My CPU is an athlon xp 2600+. iirc all my current ram is PC2100. I'd like to know what these pc numbers mean. I also need to get my head around CAS timings or w/e one of these days.