ddr2 is just the type of memory. the latest one is ddr3, however it can be very expensive. for laptops i do not feel that there is a difference.
i am planning on getting a custom laptop, u pay as much as you would pay for someting that has a name brand, except its more powerful.
this is the one i also plan on getting.
http://www.ibuypower.com/ibp/store/configurator.aspx?mid=300
scroll down and pick the parts one by one depending on the price you would like