Right now, you are nuts to buy DDR3
There is, maybe, a few % better performance with DDR3 if you buy ultra high end DDR3, but for quadrupple the price.
In 99.99% of the cases you would be better off spending the money needed to by DDR3 on just about any other component upgrade.
DDR3 will... eventually.... outrun DDR2 by quite a bit, but thats likely a good 12 months out from now.
Aledgedly Barcelona/Phenom will fly on DDR3, but anything related to those chips is pure speclation at this point.