An SSD speeds boot up and program loads. It is not a substitute for RAM. They serve different purposes entirely. Paging memory in and out of mass storage is the main source of slowdowns. An SSD can help a lot but RAM is the real solution.
My old Mac Pro has 32 gigs but the SSD makes the Computer way more fun. Since Yosemite my Mac takes longer to boot and open apps and I run everything from striped arrays with Raid 5 backups. If it were easy I would downgrade the OS to Mavericks.
I have a G4 Mac Mini running Classic with 1 gig of Ram, it pops open window and apps so fast that Apple should be ashamed of the new machines that always require more speed memory or disk space.
128 meg upgrade for my old pro are only about $1,600.00 :)