What is "Hadar Kevod Hodecha"?
Radak: The beauty of the honor of Your strength. He is honored via His strength and wonders - "v'Ikavdah b'Pharaoh" (Shemos 14:4).
Malbim: Hod is inner beauty; Hadar is external. It says about creation "Hod v'Hadar Lavashta" (104:1), for visible natural laws are Hadar, and under them is Hod - internal matters and hidden light.
Which wonders will David tell about?
Radak: Those that He did with me and will do with me. Also they 1 , whenever they receive Kel's Chesed, they tell it to many.
Malbim: I am used to talking about [Hashem's honor and wonders,] what people are already used to - unlike others, who are only Magid (tell new matters).
I.e. others of his generation, mentioned in verse 4. (PF)