First things first, it's not possible to store an application on RAM. RAM is used to RUN the application, not STORE it. That's the difference between internal memory and RAM.
Second, there should be an installer file that loads onto into an Installs folder on your memory card or phone memory that can be used to uninstall or resintall the application, though this doesn't happen all the time. If you want to keep certain applications available to switch between the memory card and the internal memory, you can keep the sis installer on a folder and uninstall/reinstall the application wherever you'd like at will, so, always keep those downloads on your PC.