DDR3 will be faster and cheaper in the long run. As long as you get a good deal on it now. Look at how unaccessible and expensive the DDR memory has become.
Even you get 8GB of PC-6400, you will barely beat 3GB of PC-16000. You also have to watch the huge price difference. The advantage of also have a board that takes DDR3 is that it will probably allow the latest processors, which can be a great upgrade solution.
Zenthar is right, if you're going Quad-Core, get the Q9450, the older models are overpriced.