FlexIO is a bus running at a max of 8GHz. When running at levels comparable to PCI Express the bus performs similar for far less power and latency.
The product formerly known as Yellowstone is now called XDR RAM - that's eXtreme Data Rate. It clocks in at a 3.2GHz data rate in its current implementation, making it around 8 times faster than DDR2. XDR can be used both for system memory and for graphics memory, and Rambus expects XDR to scale to 8GHz in 2006, making it 5 times faster than the available GDDR3.The FlexIO technology is used to connect the Cell processors to the rest of the systems and XDR RAM will be used for both system and graphics memory.
More info at The Inquirer