Right now benchmarks show no real noticeable improvement. DDR3 is new, and just like DDR2 upon release they have very high timings, and the current speeds aren't anything to write home about. I would honestly wait until DDR3 matures a bit more so the latency to speed ratio improves, and prices come down.
DDR2 that is overclocked and run using tight timings can currently achieve about the same performance as even the fastest DDR3.