What’s the difference between PC2400T and PC2400 memory

Long story short, ganged treats the memory subsystem as a single 128-bit bus, while unganged treats the memory subsystem as dual independent 64-bit buses.

Ganged favors some single-threaded software, but AMD generally recommends unganged for typical modern system configurations, especially if you are commonly multitasking multiple applications.

Basically, Gang versus unganged is more about the memory access model being used.

Buffered (also called Registered) RAM has additional hardware (a register) that sits between the memory and CPU, and will store data (buffering the data) before it's sent to the CPU. This is meant for reliability in systems that have lots of memory and lots of memory modules (think large servers), because in those systems more memory modules means more electrical demands, so buffering/registering the data reduces electrical load.

You don't really have to concern yourself with it if it's just some home machine, as it does tend to be more expensive (because it's more complex and intended for servers and machines of that class) and slightly slower (because of the memory buffering).

Also, some memory also is classified as ECC RAM, which has additional circuitry to determine if data has errors and correct them if so. It's also more expensive, and intended for reliability, and it's usually a paired feature with buffered/registered memory.