The Apple DDR systems are just a bandaid on a bigger problem.
The G4 FSB/interface to the rest of the system is only 166 MHz. Right now the G4 and the vaunted Altivec units are bandwidth-starved.
The newer Macs use 333MHz DDR SDRAM, sure, but that doesn't help the CPU at all... and in fact I believe it incurs a bit of a latency penality compared to regular SDRAM.
The faster memory on the Macs help with PCI cards that can bypass the CPU for their operations or use the memory to buffer stuff like video data or writing video data directly to a SCSI HDD or something of the sort. It all still goes through that bottleneck that is the G4 CPU's FSB, though.
If anything, the DDR memory only exposes more of what's currently wrong with the G4 as compared to any other modern CPU or system architecture.
