OpenCL SDK for Quartus Prime 15.1

Memory largely mostly depends on your target FPGA. Space usage also mostly depends on the devices that you install. If you are targetting the same device, updating to the newer versions of Quartus should not require much extra memory or space. Note that 16 GB of memory will not be enough for most designs on Stratix V and Arria 10. Also each OpenCL project takes up 1-2 GB of storage.

Altera's official recommendations can be found here:

http://dl.altera.com/requirements/17.1/

Finally, you should remember that for every version of Quartus you might want to use, you need a compatible BSP. At the end of the day, unless you are using one of Altera's reference bords, the newest version of Quartus you can use depends on the version of the BSP your board manufacturer provides.