Question:
I was doing some research into that. That seems excessive considering it can only talk to the memory at 266mhz. Do PowerPC cpu's follow the same multiplier concept as x86 cpu's? I am trying to understand why the PM didn't use higher bandwidth ram, I mean seriously the system had 20GB/s of bandwith and was fed with memory that topped out at 4.2GB/s. That seems silly to me.
The first-generation 1.6 GHz G5 used PC2700 DDR SDRAM (166 MHz, double-pumped to an effective 333 MHz,) every other 2003 and 2004 G5 (all of the PCI and PCI-X based models,) used PC3200 DDR SDRAM (200 MHz, double-pumped to an effective 400 MHz
the 2005 G5s (the PCI Express based models) used PC4200 DDR-2 SDRAM (266 MHz, double-pumped to an effective 533 MHz.) Every G5 used "dual-channel" RAM, which means that the total available memory bandwidth is double that of a single module.
This means that the 1.6 GHz G5s have 5.33 GB/s of memory bandwidth, the other PCI/PCI-X models have 6.4 GB/s, and the PCI Express models have 8.53 GB/s of memory bandwidth.
For comparison, the fastest front side bus model, the 2.7 GHz dual-socket, single-core 2004 PCI-X G5 used a 1350 MHz front side bus (all except the 2004 1.8 GHz single-CPU model used a front side bus that was 1/2 the main processor speed. The 2004 1.8 GHz single-CPU model, and the iMac G5s, used a front side bus that was 1/3 the main processor speed.) The 1350 MHz front side bus has 10.8 GB/s of available bandwidth, per processor.
This means, yes, most of the Power Mac G5 models do not have sufficient memory bandwidth to saturate even a single processor, much less two. This has been true of almost all 'consumer' level, and even most 'workstation' level computers for the better part of a decade and a half. (Basically since CPU multipliers started being used in the mid-to-late '90s.) The only system that can saturate its CPU bus is the 2004 single-1.8, which uses a 600 MHz CPU bus, for 4.8 GB/s, compared to its memory bandwidth of 6.4 GB/s. If they had used 400 MHz (effective) DDR on the 1.6 GHz model, then its processor bandwidth and memory bandwidth would have been equal. Every other model is bandwidth starved.
Correction: The last-generation 2.0 GHz single-socket, dual-core model had 8 GB/s of processor bandwidth (since both cores used one front side bus, unlike the earlier models,) yet 8.53 GB/s of memory bandwidth.
Oh, and no Power Mac G5 ever used 266 MHz RAM. (Well,
technically DDR-2 533 is really running at 266 MHz, just "double clocked" to an effective 533 MHz.)
By comparison, the Mac Pro uses 333 MHz (double-pumped to an effective 667 MHz,) DDR-2-based FB-DIMMs, in two or four channels, for a maximum 24 GB/s.
will do anything about it - probably buy in the New Year). 
