Rav Moshe Feinstein states if a person is davening in a shul which is a different Nusach than the one he has the custom to follow (for example, a man normally davens Nusach Ashkenaz, but is davening in a minyan which davens Nusach Sefard), for anything he says aloud, he should follow the Nusach of the shul. This includes Kaddish and Kedusha. Rav Moshe Feinstein also mentions that if it is not too difficult, he should recite Pesukei D’Zimra and the Brachos of Krias Shema in the Nusach of the shul, because many times these are recited aloud. Shemone Esray, which is recited silently, can be said in the Nusach that the person usually says.