Kingston offers DIMMs rated to 500MHz
Kingston is now selling DDR memory modules rated for 500MHz operation. The new DIMMs are a part of the high-performance HyperX line, which also includes modules rated at 466 and 434MHz. Unfortunately, the data sheets for the new HyperX DDR 500MHz modules aren't yet available on Kingston's web site, but the 434MHz DIMMs are rated at 2-3-3-7-1.
At $218 for a 512MB stick, the new 500MHz HyperX modules aren't cheap. However, they do come with a lifetime warranty and should be popular with overclockers looking to crank up front-side bus speeds while keeping the memory bus in-sync.