> AFAIK the PCIe models also got DDR2 RAM vs. DDR1 RAM
> in the AGP models.

Correct. It would be interesting to know whether the MPlayer performance advantage of PCIe/DDR2 G5 over AGP/DDR1 G5 is caused by the faster GPU connection like we've assumed in this thread, or by the (theoretical) doubling of the memory data transfers per clock cycle, or a combination of both.

> Would be interesting if a PowerBook G4 5,8 is faster than a 5,6 one.
> Both clock at 1,67 GHz but the former has DDR2 RAM.

The G4 CPU can only transfer data on one edge of the bus clock signal, so it utilizes DDR1/DDR2 memory as if it was SDR memory. The doubling of the theoretical data rate from SDR to DDR1 and again to DDR2 is completely lost on the G4 CPU. The memory controller of G4 Macs with DDR1/DDR2 memory however is able to transfer data on both edges, so controllers connected to the northbridge may be served on both edges, but the CPU itself cannot.
Also, you can see with koszer's lame_vmx and MPlayer results that the per-clock performance of the iBook with DDR1 is actually higher (single-digit percentage) than that of the PowerBook with DDR2. I'm not sure why that is (instead of very much same per-clock performance), but it seems to support the doubt of DDR2 trumping DDR1 here.