In order to take advantage of the bottlenecks you're getting rid of thanks to SSD, there must be a minimum CPU for it to be effective. Isn't it that on PCs that are too old, SSD wouldn't be of much use because of the bottlenecks inside the CPU? For Intel would this be i3, and what would it be for AMD? Or is this all just nonsense, and SSD is even good on a CPU from 2005?
It doesn't make any difference how fast the CPU is. It matters whether or not your motherboard has support for SATA II/III. To get the full potential out of an SSD you need a motherboard that supports SATA III which has a transfer rate of 600 MB/s, which is just above most current SSDs read times.