Even though AMD commands only a fraction of Intel's market share, Team Green may be able to leverage DDR2 support in a way that Intel can't. According to pre-launch speculation, Intel's Presler and Cedar Mill processor will be largely confined to an 800MHz FSB. If true, Intel's next-generation processors may have difficulty taking advantage of the lower latency and higher throughput high-end DDR2 offers at frequencies above 533MHz. In contrast, AMD's integrated memory controller design is theoretically capable of leveraging the additional bandwidth offered by DDR2-667 or DDR2-800—the fact that AMD motherboards now lack a front-side bus (though the term is often erroneously used) removes it as any sort of potential bottleneck.
Obviously, AMD will decide which DDR2 frequencies to officially support at launch based on market demand, pricing, and how well their future integrated memory controller handles the higher-speed DDR2 interface. If I were a memory manufacturer, I'd be hoping for something special. Aggressive high-end memory support from AMD could help sell quite a few enthusiast-oriented memory kits, and could finally push DDR2 over the hump.