krenar41 :
i know that ddr3 is faster than ddr2 but on many websites the prices of ddr2 and ddr3 are the same . I thought faster and better is always more expensive.
Many good answers have been given here explaining why the price of DDR2 and DDR3 are about the same right now.
The other underlying flaw in your thinking is just because something costs more money, does not mean it is better or faster..
Just wait until DDR2 production is ramped down in favor of DDR3, probably sometime in the next 6-12 months, and then try to buy some. As it becomes more scarce in the market, but yet a lot of people will still need it for upgrades, it will be more expensive than DDR3.