AMD recently started shipping Revision E of their 90nm Athlon 64 cores which features a small number of improvements. One of the changes is the support for SSE3 and the unofficial support for higher memory speeds than DDR400.
AnandTech decided to test the new Athlon 64 Revision E on the DFI LANParty UT nForce4 Ultra-D motherboard with the 704 BIOS installed. They compared system performance in various benchmarks with the memory clocked at DDR400, DDR436 and DDR480. In most cases they only noticed a minimal performance increase.
Based on the tests that we’ve seen here today, AMD’s reluctance to move to higher bandwidth DDR2 offerings makes a lot more sense. The plain fact of the matter is that at the current clock speeds at which the Athlon 64 and X2 line are running, most desktop applications see virtually no benefit from higher bandwidth memory. It is possible that server usage models may show a greater performance boost, but it is highly unlikely for a mission critical server to be equipped with anything that isn’t an officially supported standard - especially memory.