If you're running out of resources it has less to do with RAM and more with the CPU. Also, it's not the samples or the quality of the instrument that determines how much processing power it requires but the quality of programming. Some seemingly insignificant plugins might hog a lot of resources if not optimized properly on the programming level.
That being said, I find it difficult to believe it'd be a CPU-related problem. You could always try narrowing the problem down. e.g:
1) test out different plugins. do they all crackle?
2) test out different soundcards if possible. similar problems?
3) test out different DAWs, like demo version of Reaper. same problems?