My Desktop-Computer is about 8 years old and I use it mainly for work (Darktable, Gimp, Inkscape, some video & music editing) and some gaming (e.G. Layers of fear, ...). I have some problems with the performance, e.G. I couldn't use the Affinity-programs by serif - drawing a line took about 5 seconds. And Layers of Fear is just almost fluent on the lowest settings. You should know, that I use both Windows 7 and Bunsenlab Linux (dualboot).
I wonder if it would be worth it, to invest into some new hardware (additional RAM, better GPU, SSD-drive) or if I would be better off, if I bought a new Desktop/Laptop.
I am looking forward to your answers and hope you can ignore my lack of english-skills. If you need anymore information about my setup, feel free to ask.
We're at a disadvantage since you did not share any specifications of the current PC, but in general - do not spend money to try and boost performance. The computer is very old and outdated and it will be a waste of money. At best, you would see only very modest gains.
Go with a new system if you can afford it. For editing HD video, you'd want an Intel Core i7 with nVidia graphics, such as GTX 1060 or 1070 for instance, 16GB RAM minimum, SSD boot drive, etc.