Overclocking the core has more of a noticeable increase than overclocking the memory, that's all. Both increase the speed.
As for my overclock, I think I just got a good overclockable card. I wouldn't expect to get as high as 600MHz, I certainly didn't. Also I think my memory will probably go higher, but it fails the driver test in RivaTuner so it won't let me increase it.