Based on the dual channel system board designs, memory modules are installed on two separate channels, each with it’s own access route to the memory controller. The dual memory channel design doubles the peak bandwidth of single memory channels. Best results occur when a matched-pair memory kit is used ensuring that the modules are identical.
For more info, visit:
*www.cpuplanet.com/features/article.php/1587771
BTW regarding DDR2, the thing is that a 400 MHz ddr ram is better than a 533 MHz DDR2 ram as of date. Moreover, if you are getting an amd 64 bit processor, it is more than enough if you get a 400 MHz ddr ram. this would give you a great performance.