Virtual RAM needs to be written to and read from the hard disk. The hard disk is by far the slowest component in your computer. Avoiding access to the hard disk will speed up everything. Poser (as any other program) benefits from additional RAM. It can store everything it needs in memory and still has some memory over to use as a cache to temporarily store things. Nowadays textures are very large and having them in memory will speed up things.

With regards to an SSD I can only say it will bring an improvement in performance but not as much as sufficient RAM. When buying an SSD be sure to buy one big enough to contain the OS, all your executables, and all your temporary files. Personally I would advise buying at least 500 GB (more is better).