Celeron vs Athlon vs Pentium

I’m a network tech for an office w/about 20 PC’s. They have a variety of processors from Celeron 1.07 Ghz to Athlon’s to P-4 3.0 ghz. I’m looking for some kind of ‘bench-mark’ to tell which PC’s need to be upgraded/replaced/raped for spare parts. Can anyone provide info on differences between these? I have heard that a celeron is basically a ‘broken’ pentium. Anybody have any insight on rating/explaining differences between celeron/athlon/pentiums? Thanks!