MacBidouille (temporarily housed at http://www.hardmac.com) posted a graphic from an American RAM Distributer which advertises DDR Ram for XServe and the "New PowerMac".
Of note, it lists rates of 266Mhz and 333Mhz, while the XServe only supports 266Mhz DDR RAM.
Now, more often than not, ads such as these from 3rd party vendors represent educated guesses by the vendors than any true leaks.
