Well, im sure there is people out here who explain things better, but it divides the processing between two cores, thus letting each of them do their job a little bit faster. Even though the latter on has a "lower ghz" then the first, it should still be better because it is two cores, instead of one. So basically, to answer your question, yes. It should be a noticeable difference. Games should run smoother.
definitely the Dual Core one. the frequency of 1.73 GHz is actually the frequency of only one of the cores. The other one works on the same freq. which means that in total you'll have 3.56 GHz.