The performance difference between DDR2 and DDR3 is minimal in comparisons to other upgrades you could make.
Assuming your CPU is not going to cap the performance of the 6950, the GPU update seems like it would be better.
However I have to ask you; if the 5850 is running largely without issue why bother upgrading? Sounds like you want to upgrade for the sake of it rather out of necessity!
Perhaps consider picking up a shiny SSD if you really want to spend the money?