nss vs. pam

Can anyone explain the interaction between nss vs. pam? I just discovered the nss_db package and it looks like what I need, more than pam. I want to create a system account for every user, but I want to avoid linear search lookups in a large plaintext /etc/passwd file. It looks like nss_db might solve that problem. Since I don't care about external authentication, can I remove pam, to speed things up?