I did some research and found out the following:
DDR2 RAM is not backwards compatible with DDR. Check the system profiler or "About this Mac" to see which RAM you have. The G5 quad has DDR2
The frequency of the RAM, e.g. 533, 667, 800 MHz is the certified or maximum speed and it is backwards compatible, e.g. PC2-8000 is backward-compatible for PC2-4200, PC2-5300, and PC2-6400. PC2-4300 is PC-4200 that has been tested at higher than standard rates.
Old memory modules are often more expensive than newer. There is no reason to buy PC-4200 if you can get PC-6400 for the same price. But you increase your chance to be able to use the same RAM in a newer computer. Higher performance DDR2 RAM is compatible with lower performance and the higher performance module runs at the lower module's frequency. Using lower performing DDR2 memory in a system capable of higher performance results in the bus running at the rate of the lowest performance memory in use.
The biggest question might be whether or not to go for Buffered, Fully Buffered, Registered, Unregistered, or ECC nor non-ECC.
EEC adds correcting minor errors and detecting major errors for better reliability. ECC is a bit more expensive but I prefer the reliability. You can use ECC memory even if your system does not support it, but don't mix ECC and non-ECC modules within the same memory pair or memory bank. Modules with ECC are identified by an additional ECC in their designation., e.g. PC2-4200 ECC
Registered RAM means "buffered", and unregistered means "unbuffered". It provides more reliability and scalability but is somewhat slower than unregistered RAM. If you install registered RAM in a memory bus that does not support it than it will not be used. You can save some money here, but some server platforms use registered RAM. You cannot mix registered and unregistered RAM in your system. Unregistered RAM may be identified by an additional U in the designation. PC2-4200R is registered.
Fully buffered modules, which are designated by F or FB do not have the same notch position as other classes. Fully buffered modules cannot be used with motherboards that are made for registered modules, and the different notch position physically prevents their insertion.
The G5 quad supports ECC. However, my G5 Quad shows PC-4200U, which means memory at 533 Mhz, unregistered and non-ECC. It was probably the cheapest at the time.
I'm going to put PC2-6400U ECC into my G5, which is cheaper than PC2-4300U and I can also put it into an April 2008 iMac, and let you know the results if it does not work.
best wishes,
Markus
