I have a 642. So far, I have installed a Wolff 8 lb mainspring and 14 lb rebound spring. I have both 13 and 12 lb rebound springs on order, as it is still a bit too stiff for my liking.
If you go this route, be sure to grind the ends of the rebound spring flat, as is the factory rebound spring. Otherwise, it is difficult to install even with a dedicated gunsmithing tool. Grinding is easy with a common drill and grinding stone attachment.