Legit Reviews takes the time to compare DDR2 to DDR3 in terms of system performance.

We set out to see if there was a significant difference between DDR2 and DDR3 memory on both of the Intel P35 Express based ASUS motherboards and found that there really isn't. Right now there is no clear call as to what would be better for gamers and enthusiasts as it depends on your tastes. As with all new technology it will only get better as time goes on and with memory companies hitting ~1,800MHz on the initial 512Mb DDR3 IC's it shows great promise for the new memory type.

Intel P35 Chipset: DDR2 Versus DDR3 Memory