If you bench a 7600GS on DDR2 against a 7600GS on DDR3 at stock speeds you will see very little to no difference in performance in 3D Marks, quake 4, oblivion, etc. etc. clock them up to the max they will go and you will still only see a very slight difference, take a 7600GT on DDR3 and clock the core down to the same speed as a 7600GS on DDR2 and you will again see very little difference.
I can't say the same is true for ATI cards, but I have extensive experience overclocking nVidia cards in the 5xxx through to 7950GX2 range.
Just whatever you do... stay away from cards with a 64bit DDR memory interface!