Um, if you went from 2x1GB @ CL=3 to 2x512MB @ CL=2 you sure would notice a difference - your system would be slower by a gig of RAM!
People wrongfully think that CAS has a big effect on system performance; the difference between 2 and 3 is negligible. If you don't believe me then put in the 2x512 and run 3dmark05 at CAS=2 and then at CAS=3 and you'll see how little difference there is in the score. Then put the 2x1GB back in at CAS=3 and you'll find a huge increase in your score.