First - run the performance report (in admin reports)
150 people online at the same time *could* easily overpower a 6GB server - it depends what they are doing. 150 people all doing a quiz is entirely different from 150 people reading documents. 6GB isn't a lot of memory.
What operating system is it? How was moodle installed? How did you tune the web server and the database server? What software are you running to monitor performance and what does it tell you? And so on...
(I hope it's not Windows...)