It isn't worth trying to upgrade/'fix' that computer, which is well over 10 years old, so, yea, not even close to adequate for real-time video compositing
then next question tends to be, well what do you need? and the answer to that is.. it depends... You could get a decent system from 5 years ago, and that be fine. Or, you could spend US$5,000 on a brand new, top notch system of today and still overwhelm it, depending on background processes and settings. None of us can tell you what your priorities and budget are. Or how long you want the system to last? for example, do you want/need the system to be able to support 4K streaming in a few years?
etc