I was wondering if you could clarify a few halachot concerning dress during tefilla. Firstly, what is the halacha about wearing a belt, or gartel; does a regular leather belt suffice etc.? Secondly, does one need to wear a jacket during tefilla? And lastly, I have heard that one should have a special kippah for tefilla, is this required?

There is no obligation (though for some it is a custom) to wear a "gartel" during tefilla (Ishei Yisrael 10 7). The “pshat” understanding of the halacha in the Shulchan Aruch (91,5) is that in today’s context there is no obligation to wear more than a kippa when davening. The Mishna Berura (91,12) following the Chayei Adam and in line with the “pshat” of the Shulchan Aruch, bases the obligation to wear a hat when davening on the prevailing custom whereby someone would always wear a hat when appearing before important people (the same logic applies to wearing a jacket) . Today this is of course, not the case. Both the Tzitz Eliezer and the Yabiya Omer recommend (they do not write that this is obligatory) wearing in shul a kippa that covers most of the head (this is the special kippa to which you refer). Outside of shul, a kippa that doesn’t cover most of the head is sufficient as long as the head would be described by an observer as being covered (Iggrot Moshe Orach Chayyim I 1).