What do we mean when we praise hashem as “Hakadosh”, “the holy G-d” (in the amidah), which is one of the blessings that we say (hakeil ha-kadosh)? Does it make a difference to me how holy/great Hashem is?
First of all, it does make a difference. We are instructed to emulate Hashem, and one of the attributes the Midrash teaches us to emulate is His holiness (Sifri, Eikev). Therefore, his holiness is relevant to us. Of course, there is much room to understand how we are to emulate this attribute, but we will not go into this matter at this stage.
However, the point in the Amidah is not that it makes a difference, but rather to praise Hashem for His greatness and His holiness. The first three blessings are blessings of praise, and they introduce our faith in Hashem, His greatness, His might, and His holiness, before we begin to make requests.