On one level, the question is impossible to really answer, hence why you haven't found one. As everyone writes differently and edits parts differently, there is simply no easy answer.
More information would also be helpful. Are you seeking to run VSL full libraries, or VSL SE libraries? The SE libraries are much less demanding than the full libraries, and you could probably load a fair amount of samples in 4 gigs (SE 1, 1+, 2 and 2+ fill up about 8 gigs of RAM with all patches loaded). However, with the full libraries, plus those of other companies, having only 4 gigs of memory would be your first bottleneck. It is not uncommon to have 32 gigs of ram on a computer where many samples will be loaded.
What speed is your hard drive, and how many hard drives do you have? If your hard drive is 5400 rpm, that slow speed would cause problems. Also, if you are trying to run your o/s, recording, and streaming samples from one drive, that would be a significant bottleneck. If using one computer, most often 3 hard drives are used: one for o/s and programs, one for recording audio, and one for streaming samples.
In addition, as you mention, beyond a certain point, more processing power is required as well, to run all the various processes. In general, sample streaming requires ram, fast hard drives, and a higher cpu core speed. For running multiple effects, and audio processing, having more cpu cores is more important than the speed of the cores.
Note that using SSDs for sample streaming is also helpful: samples load much faster, more smaples can be streamed, and with VSL, buffer sizes can be lowered, which helps conserve memory usage.