Hard drive differences

I am upgrading a number of desktop systems to Windows Professional. I am going to have to replace some of the hard drives (current ones are to small)Need some help in determining whats what.
What is the difference between a ATA/100 and ATA/66?
Isa 7200rpm faster that a 5400rpm?
Which one configuration is better and why?
I don’t need large hard drives but the 2g ones I now have is some of my machines are to small.