iXBT Labs posted a review on Phenom II Processors and DDR3 Memory. Here's an excerpt:

What concerns performance comparison with DDR2 and DDR3 memory, DDR3-1333 brings only an advantage in memory bandwidth, while random seek latencies are higher than in DDR2-1066. DDR3-1600 is better than DDR2-1066 in all parameters already. And vice versa: it's clearly a bad idea to use DDR3-1066, as its bandwidth is similar to that of DDR2 at the same frequency, and its latencies are higher. By the way, it unexpectedly turned out that Apacer modules (DDR3-1333) worked well at 1600 MHz, their voltage raised only to 1.6 V.

But the real contrast between computers with DDR2 and DDR3 memory appears in the overclocked mode. As we already noted, in order to overclock our Phenom II X4 810 we had to compensate memory frequency to bring it closer to DDR2-800, even if its typical mode was DDR2-1066. There is no such restriction for DDR3 memory, and its top stable frequency is a tad higher. However, this difference will be smaller, if you deal with a processor that has an unlocked multiplier (so that you can preserve DDR2-1066 mode in overclocking). It's also obvious that an overclocked processor needs faster memory (and can use it effectively), so practically in all tests the difference is higher that we can see comparing DDR2-800 and DDR3-1600 results at the typical frequency, especially for multi-threaded access. So DDR3 memory is more justified for enthusiasts and overclockers than for those users that employ typical modes.

To all appearances, the evolutional strategy offered by AMD is adequate to the real situation on the memory market. On one hand, a small advantage in typical modes does not justify the price difference between DDR2 and DDR3 memory kits of same size and class (at the time this article was written). On the other hand, PC manufacturers can buy standard DDR3 modules for much better prices than those offered in retail stores. It seems that this migration becomes expedient even from the economic point of view, especially considering the existing performance advantage. Another category of users that might be interested in DDR3 memory (for a new computer) is consistent overclockers. Processors operating at increased frequencies may squeeze a better potential from high-speed DDR3 memory. As a result, a user may get higher performance. The difference is not big, but it's not that small either. You would get a similar performance gain from a CPU with a 100-200 MHz higher clock rate.