If you currently have 2X4, you can add two more 4G modules to go to 16.  Doing so will not markedly increase the speed of the system, however - it may somewhat improve things, but don't expect major boost.

The single slowest device in the system, assuming it has a conventional hard drive, is the drive subsystem.  Running the operating system and your rendering software from a solid state drive is the single thing you can do that will dramatically boost the system performance.

Thanks for the info ejn63. Can I install a solid state drive in the current machine or would I have to replace it?

You can easily install a solid state drive in the system --you need a standard 2.5" SATA drive (if you want to mount it, you'll need a 2.5" to 3.5" adapter bracket).