A 4GB DDR2 (Bus 667) RAM is better or a 2GB RAM DDR3 (Bus 2000) is better?

No doubt DDR3 is latest compare to DDR2.But you can see their capacity difference by half.I always suggest 4 GB DDR2 better than 2 GB DDR3.

4GB DDR2 (Bus 667) RAM

-2GB is not enough to even run windows.

But at the same time the 2GB one more powerful

I would just buy 2 x 4GB for 1444mhz ddr2

that would cost up to 40-100 $

4gb is better.But you can't compare a DDR2 Ram with a DDR3 RAM

as these two require two different motherboards (thus, different processors), you can't compare them.