DDR3 memory is now starting to show its true colours. Yes, at this stage it's more expensive, but for the ultimate system experience, for those who want top performance and overclocking, then DDR3 based systems are the way to go. DDR2 based systems just can't clock as high due to a bottleneck in the DDR2 bus; 1200MHz is the limit, and unless something happens (die shrinks or super high yields) DDR3 is set to take over quite convincingly as the overclocking memory of choice.
Kingston's HyperX memory range has really come of age; its performance is extremely impressive, as well as its ability to overclock. After all, it may be rated for 1800MHz, but remember that 1333MHz is the official speed recognized by JEDEC. NVIDIA has its SLI ready recommendations which are overclocked profiles, so beyond 1333MHz is an overclock. With that said, 1800MHz is a quite a massive jump.
Kingston already has its 2000MHz line coming soon, so stay tuned for a review of this up against the SLI-ready memory. How well it performs may impact your buying decision.