Please note that the advantage of Hashtable over generic Dictionary for insert and search operations demonstrated here is actually because the tests are based on NON generic IDictionary interface, so each insert or search action is accompanied with check for the key type. For more information see the Sean's comment below. (Thanks Sean!) Without that Dictionary seem to perform better than the Hashtable. The rest of the test conclusions will not be affected. So to summarize, generic Dictionary is the absolute winner for insert and search operations if used with generic dictionary interface or directly.
This is a sequence of tests comparing the performance results for four different implementations of IDictionary and in particular generic Dictionary, generic SortedDictionary, the old non-generic Hashtable and generic SortedList.