A little more detail on the scratch disk...
When the system can not fit everything it is working on into physical RAM, it swaps out pages of memory to a physical drive, and recalls them as needed. The more page outs you have, the slower things will become. Of course this is also determined by the speed at which those pages outs can be written to and read from the drive.
So, the more RAM you have, the less page outs. And the faster your drive, the less time those page outs will take.
IIRC, PS has an optimized scheme regarding handling its own scratch disk operations instead of just blindly handing it off to the OS.
You may notice a little speed increase in day to day operations with the extra RAM. In the case of the SSD, after booting your PC for the first time, and playing with it for a few minutes, you'll wonder how you ever got along without one.
Again if your motherboard does not have a SATA interface, spend the extra hundred and get the PCIE version (assuming you have an open slot).