Unfortunately, according to Rob Art Morgan who has tested this, the new PowerMacs from Apple that use DDR (double data rate) memory like its Xserve rank-mount unit cannot access the memory any faster than the cheaper and slower SDRAM found in the previous system arch. A controller limits the data rate to 1 Gbps, while DDR could work more than twice as fast. Unfortunately, this makes mincemeat of the architecture, as it bus/memory-bounds 2D and 3D graphics and rendering.
In practical terms, my chart showing the Apple vs. Dell server comparison is now pointless: the new Macs can't compete with the same-price or cheaper Dells because the Macs are just as slow as the old Macs. You can boost the processor speed somewhat (jump to dual 1.25 GHz), but you can't extract nearly enough speed from the memory.