Advantages of DDR3?

They perform similarly, however DDR3 runs at a lower voltage and draws less power because of that, most modern processors like the i7 and new Phenom II's have DDR3 memory controllers built in so DDR3 is becoming more and more common. The price of DDR3 is dropping so it might be a good idea to get one that supports it so that in the future you have access to cheaper upgrades but right now its a bit more expensive.

Actually, DDR3 is both faster and lower power. It has similar latency to DDR2, but higher data rates. It isn't worth it without an integrated memory controller (such as the one found on the Phenom II or Core i7 series CPUs) though, because the memory can be bottlenecked by the older FSB architecture and it doesn't give as much of a gain as it does on the newer IMCs.