I just ran a test where I had the Sample Tank Electric Grand with all effects on in Studio One 5.4.2. I watched my resources in the task manager. Closing the GUI, believe it or not, reduced the CPU performance significantly. I then added a second track with the Hammond B-3X. Everything ran fine, no pops, no clicks, no crackles. I tried to break it, so to speak, but never really saw the CPU in the task manager go more than 30%. It rarely went above the 2.8 GHz base speed, though when I was recording with the Electric Grand, it sometimes went up to 3.5 GHz.
Memory hardly budged. With the OS and others at about 6GB Ram usage, opening studio one and adding these tracks only bumped it to about 7 GB. I'm a hobbyist with a home studio and I doubt I'll exceed 8 tracks. I think I'm safe with 16 GB for now.
I think I'm going to hold off on a new computer. I'm going to continue optimizing this one and keeping CPU draining apps and GUIs down to a minimum.
By the way, if you turn off the effects rack in SampleTank 4 on your instrument, the CPU load decreases significantly. However, with all the other optimizations, I was able to leave the effects on while recording.