It is not always about how much RAM that you have got.
You need to look at the motherboard that is installed and look at its' specifications. For instance what is the 'fastest' and or best CPU for your particular requirements, that you can install. The same with the video card. The same with the RAM. (can you install faster RAM whilst maintaining stability?)
Also, installing a SSD in lieu of a HDD for your OS, will speed up things.
If you are using the external HDD to store what you are working on, then is it USB 3.0 (faster than USB 2.0)?
You will have to research for what you want and how much you want to spend.
In the end it will be a compromise between how much do you want to spend to upgrade your existing system, to obtain the result that you want. (starting with what the motherboard can handle, unless you replace the motherboard and then you will have to do a total rethink as to what you need)