Intel E series vs. I5

Seems like in the good old days, you could simply look at the clock speed and know which computer was faster.  But now, as we try to buy the right computer for office users, we are confronted with a wide array of confusing choices.  Specifically, the user wants Lenovo desktops, and at the low end, I see all kinds of models with E3400, 5500, 5700, 5800, 7500, and more--yet the clock speeds are all over the map, ranging from 2.6 to 3.2.  So in the E series, when you're trying to buy the fastest one, do you look at the "3400" or "5500", etc. to determine which is faster, or do you go with the faster clock speed...or how exactly DO you know which is fastest?

And then, for more money, they offer a few models that have I5 or I7 processors.  Would all of these be faster than the E series?  And if I have a choice of models with an I5, for example, can I just go with the fastest clock speed to know I'm getting the fastest computer?