Need help with chip speed comparisons on desktop computers

I have some confusion re: 'core 2 duo' vs. 'dual core'.  I need to choose from one of these for new PCs running office applications on a network.

Core 2 duo E6400 2.13ghz

Dual core E2200 2.2 ghz

Core 2 duo E6550 2.33 ghz

Are these three progressively faster, in this order, due to their processor speeds alone?  Or, does the 'core 2 duo' vs. 'dual core' design also make a difference in the speed?

Ideally I need to see them in order from slowest to fastest---because I don't know whether processor speed alone tells me the correct order or not.  TIAStart Free Trial