Read the changelogs from 1.5.3 to 1.5.5 and see if you need to take additional steps

ngircd has the concept of a global password. Distribute this password and you’ve got a simple psk based acl mechanism

what error message are you getting?

It all depends on what you do with the box, how the applications you run utilize the cpu (single vs multithreaded/multiprocess but also stuff like cpu cache utilization. if you profile the workload that you want to run (see if its cpu, memory or io bound for instance) will help you in figuring out what works best for you.