DDR (PC3200) and DDR2 RAM (PC2-4200, PC2-5300) are different electrically and physically - they are not compatible with each other.
You have to specify whether you have a Dual-Processor 2.3 GHz G5 (PCI-X bus, DDR PC3200 RAM)
or a Dual-core 2.3 GHz G5 (PCI-e bus, DDR2-4200 RAM) which was built after Oct. 2005
You cannot 'improve' the memory speed on either machine, you are stuck with what Apple designed it for.
That said, you need a MINIMUM of 2 Gb for any kind of professional software to perform to its potential, and 4 Gb would not be out of place for graphics, video or audio work.
I advise you to stick with a reputable memory dealer who knows Macs and can guarantee compatibility with your specific model Mac. BestBuy and other discounters will happily sell you the wrong thing if you order it, and it's your time and money wasted to send it back.
To the original question:
The speed of RAM performance is determined by the memory bus speed (which is set by the motherboard, the higher the better) and by the latency of the RAM (Lower the better, limited by the capability of the RAM module, and the capability of the motherboard to run the RAM at lower latency).
Whether 2 types of RAM are can be compared directly is a little tricky. Yes, the DDR RAM is a 400 MHz module and the DDR2 RAM is a 667 Mhz. But the 400 MHz RAM runs at a CAS latency of 3, while the 667 MHz DDR2 RAM runs at CL 5, which cuts its speed advantage down a bit.
The FB-DIMMs in a MacPro are another story entirely. The FB-DIMMs have an advanced memory controller chip on the DIMM, so they offload some of the management from the CPU, allowing for larger memory sizes, highly reliable Error Correcting, and easier motherboard design. The tradeoff is heat, and latency. FB-DIMMs run between CL 7 and CL 9 depending on which slot they are in on the riser. Slots 3 and 4 are slower than slots 1 and 2, because all requests are first sent to the lower slot, and then forwarded by the controller on the first FB-DIMM to the upper slot, which causes added latency.
G5 Dual Processor = DDR 400 MHz @ CL3
G5 Dual-core = DDR2 533 MHz @ CL5
MacPro = DDR2 FB-DIMM 667 MHz ECC @ CL7
MacPro Penryn = DDR2 FB-DIMM 800 MHz ECC @ CL7 (? possibly, we don't know the latency yet)
When you get into comparing the speed effects of DIFFERENT sizes of different types of RAM, then all bets are off. On top of bus speed and latency, it depends on how much RAM, how many programs you are using, how big the data is, how well the programs and the OS exploit more RAM, etc. There is very little way to make any informed guesses with so many variables.
