Hello and welcome to Tom's Hardware Forums.
DDR3 is faster and therefore supposedly better.
DDR3 will not fit in a DDR2 or DDR slot even if you hit it very hard.
When DD3 came out, DDR2 prices went through the roof. This industry has a habit of forcing us to spend money - even when we don't have any - by rachetting up the hardware requirements and making perfectly good products history overnight.