Getting to 2000MHz wasn't an issue for us. At 4GHz on our CPU we used the dividers to achieve 2000MHz exactly and also set the timings correctly. During our testing we didn't have a single issue with the modules, which is fantastic.
Getting higher wasn't easy, however. While we did get into Windows at 2010MHz - 2020MHz performance was a bit flakey and we opted to not bother pushing any more voltage to try and achieve any higher.
Important Editor Note: Our maximum overclocking result is the best result we managed in our limited time of testing the memory. Due to time constraints we weren't able to tweak the motherboard to the absolute maximum and find the highest possible FSB, as this could take days to find properly. We do however spend at least a few hours overclocking every motherboard to try and find the highest possible overclock in that time frame. You may or may not be able to overclock higher if you spend more time tweaking, or as new BIOS updates are released. "Burn-in" time might also come into play if you believe in that.