check these links,
http://www.overclockercafe.com/Articles/DDR_vs_DDR2/
read the whole article .. they have done complete benchmarking of both the rams there.
http://support.intel.com/support/mot.../CS-012038.htm
btw whats the config that u r going for ? and remember ddr2 is almost double the price of ddr making it very expensive.