Would 3G of DDR2 memory be better than 2G of DDR3?

Im getting a new computer, as i can get a new computer which is better in almost every way to my old computer for less money than it would cost to fix my old computer. However the only thing bad about the computer which im thinking of getting is that it has less RAM. But that doesn't matter, because i can just cannibalize that from my old computer. However i know little about DDR3 RAM. Whats the deal? Does it represent some kind of technological breakthrough which makes it more effective than a larger amount of DDR2 RAM? What should i do?