How Much Unix Knowledge For Helpdesk Staff?

Shisha asks: "I have to train IT helpdesk staff to be able to solve UNIX related problems. The trouble is I don't really know what will be most useful for them. I can't teach them all about shells, processes, deamons, sendmail, Xwindows etc. because that's just too complex. Another problem is in the slight differences between AIX, Solaris, Irix and Linux. The idea is that they should be able to solve simple problems, so that the really qualified people won't have to waste their time on them. We are talking about 16,000 potential users, on 5-10 multiuser servers and about 300 Xwindows workstations, so the number of problems daily is high. Also what commands would they need with root privileges (through sudo)? So the question is what should they know and what access rights should they have."