One thing to be aware is that for older CPUs, the limiting factor is often compatible motherboards. The original ones are more likely to have died than CPUs from the same generation, and new ones aren't being made.
I was thinking that an old motherboard may require a BIOS update for a relatively newer, but socket compatible CPU. I remember having to ebay in a random CPU just to able to turn on a motherboard to update the BIOS for the better CPU I bought earlier.
