Also majorly depending on the chipset used by the motherboard, you may or may not get signifiicant improvements on using the higher speed DDR RAMs.
The motherboards would specifiy the FSB (Front Side Bus) clock speed, but even then if you look at independent reviews and benchtests, some motherboards may support the latest & greatest memory speeds but you would be getting less than 1% improvement in performance for paying heaps extra for the high speed RAM.
Try going to sites like
www.anandtech.com or
www.tomshardware.com for VERY technical & detailed reviews.